# Source code for espnet2.asr.transducer.rnnt_multi_blank.utils.cuda_utils.gpu_rnnt_kernel

```
# Copyright (c) 2021, NVIDIA CORPORATION. All rights reserved.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
# http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
#
# Copyright 2018-2019, Mingkun Huang
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
# http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
import math
import torch
from numba import cuda
from espnet2.asr.transducer.rnnt_multi_blank.utils import rnnt_helper
GPU_RNNT_THREAD_SIZE = 256
[docs]@cuda.jit(device=True, inline=True)
def logp(
denom: torch.Tensor,
acts: torch.Tensor,
maxT: int,
maxU: int,
alphabet_size: int,
mb: int,
t: int,
u: int,
v: int,
):
"""
Compute the sum of log probability from the activation tensor and its denominator.
Args:
denom: Tensor of shape [B, T, U] flattened. Represents the denominator of the
logprobs activation tensor across entire vocabulary.
acts: Tensor of shape [B, T, U, V+1] flattened.
Represents the logprobs activation tensor.
maxT: The maximum possible acoustic sequence length.
Represents T in the logprobs tensor.
maxU: The maximum possible target sequence length.
Represents U in the logprobs tensor.
alphabet_size: The vocabulary dimension V+1 (inclusive of RNNT blank).
mb: Batch indexer.
t: Acoustic sequence timestep indexer.
u: Target sequence timestep indexer.
v: Vocabulary token indexer.
Returns:
The sum of logprobs[mb, t, u, v] + denom[mb, t, u]
"""
col = (mb * maxT + t) * maxU + u
return denom[col] + acts[col * alphabet_size + v]
[docs]@cuda.jit()
def compute_alphas_kernel(
acts: torch.Tensor,
denom: torch.Tensor,
alphas: torch.Tensor,
llForward: torch.Tensor,
xlen: torch.Tensor,
ylen: torch.Tensor,
mlabels: torch.Tensor, # [B]
minibatch: int,
maxT: int,
maxU: int,
alphabet_size: int,
blank_: int,
):
"""
Compute alpha (forward variable) probabilities over the transduction step.
Args:
acts: Tensor of shape [B, T, U, V+1] flattened.
Represents the logprobs activation tensor.
denom: Tensor of shape [B, T, U] flattened. Represents the denominator of the
logprobs activation tensor across entire vocabulary.
alphas: Zero tensor of shape [B, T, U]. Will be updated inside the kernel
with the forward variable probabilities.
llForward: Zero tensor of shape [B]. Represents the log-likelihood of the
forward pass. Returned as the forward pass loss that is reduced by
the optimizer.
xlen: Vector of length B which contains the actual acoustic sequence
lengths in the padded activation tensor.
ylen: Vector of length B which contains the actual target sequence
lengths in the padded activation tensor.
mlabels: Matrix of shape [B, U+1] (+1 here is due to <SOS> token
- usually the RNNT blank). The matrix contains the padded target
transcription that must be predicted.
minibatch: Int representing the batch size.
maxT: The maximum possible acoustic sequence length.
Represents T in the logprobs tensor.
maxU: The maximum possible target sequence length.
Represents U in the logprobs tensor.
alphabet_size: The vocabulary dimension V+1 (inclusive of RNNT blank).
blank_: Index of the RNNT blank token in the vocabulary.
Generally the first or last token in the vocab.
Updates:
Kernel inplace updates the following inputs:
- alphas: forward variable scores.
- llForward: log-likelihood of forward variable.
"""
# // launch B blocks, each block has U threads
b = cuda.blockIdx.x # // batch id
u = cuda.threadIdx.x # label id, u
T = xlen[b] # select AM length of current sample
U = ylen[b] + 1 # select target length of current sample, +1 for the blank token
labels: torch.Tensor = mlabels[
b
] # mb label start point, equivalent to mlabels + b * (maxU - 1)
offset = b * maxT * maxU # pointer indexing offset
# alphas += offset # pointer offset, ignored since we explicitly add offset
# Initilize alpha[b, t=0, u=0] for all b in B
if u == 0:
alphas[offset] = 0
# sync until all alphas are initialized
cuda.syncthreads()
# Ordinary alpha calculations, broadcast across B=b and U=u
# Look up forward variable calculation from rnnt_numpy.forward_pass()
for n in range(1, T + U - 1):
t = n - u
if u == 0:
# for t in range(1, T) step to initialize alphas[b, t, 0]
if t > 0 and t < T:
alphas[offset + t * maxU + u] = alphas[
offset + (t - 1) * maxU + u
] + logp(denom, acts, maxT, maxU, alphabet_size, b, t - 1, 0, blank_)
elif u < U:
# for u in range(1, U) step to initialize alphas[b, 0, u]
if t == 0:
alphas[offset + u] = alphas[offset + u - 1] + logp(
denom, acts, maxT, maxU, alphabet_size, b, 0, u - 1, labels[u - 1]
)
# for t in range(1, T) for u in range(1, U) step to compute alphas[b, t, u]
elif t > 0 and t < T:
no_emit = alphas[offset + (t - 1) * maxU + u] + logp(
denom, acts, maxT, maxU, alphabet_size, b, t - 1, u, blank_
)
emit = alphas[offset + t * maxU + u - 1] + logp(
denom, acts, maxT, maxU, alphabet_size, b, t, u - 1, labels[u - 1]
)
alphas[offset + t * maxU + u] = rnnt_helper.log_sum_exp(emit, no_emit)
# sync across all B=b and U=u
cuda.syncthreads()
# After final sync, alphas[b, T-1, U - 1] + logprobs[b, T-1, U-1, blank]
# + denom[b, T-1, U-1] gives log-likelihood of forward pass.
if u == 0:
loglike = alphas[offset + (T - 1) * maxU + U - 1] + logp(
denom, acts, maxT, maxU, alphabet_size, b, T - 1, U - 1, blank_
)
llForward[b] = loglike
[docs]@cuda.jit()
def compute_betas_kernel(
acts: torch.Tensor,
denom: torch.Tensor,
betas: torch.Tensor,
llBackward: torch.Tensor,
xlen: torch.Tensor,
ylen: torch.Tensor,
mlabels: torch.Tensor, # [B, U]
minibatch: int,
maxT: int,
maxU: int,
alphabet_size: int,
blank_: int,
):
"""
Compute beta (backward variable) probabilities over the transduction step.
Args:
acts: Tensor of shape [B, T, U, V+1] flattened.
Represents the logprobs activation tensor.
denom: Tensor of shape [B, T, U] flattened. Represents the denominator
of the logprobs activation tensor across entire vocabulary.
betas: Zero tensor of shape [B, T, U]. Will be updated inside the kernel
with the backward variable probabilities.
llBackward: Zero tensor of shape [B]. Represents the log-likelihood
of the backward pass. Returned as the backward pass loss that
is reduced by the optimizer.
xlen: Vector of length B which contains the actual acoustic
sequence lengths in the padded activation tensor.
ylen: Vector of length B which contains the actual target sequence
lengths in the padded activation tensor.
mlabels: Matrix of shape [B, U+1] (+1 here is due to <SOS> token
- usually the RNNT blank). The matrix contains the padded target
transcription that must be predicted.
minibatch: Int representing the batch size.
maxT: The maximum possible acoustic sequence length.
Represents T in the logprobs tensor.
maxU: The maximum possible target sequence length.
Represents U in the logprobs tensor.
alphabet_size: The vocabulary dimension V+1 (inclusive of RNNT blank).
blank_: Index of the RNNT blank token in the vocabulary.
Generally the first or last token in the vocab.
Updates:
Kernel inplace updates the following inputs:
- betas: backward variable scores.
- llBackward: log-likelihood of backward variable.
"""
# // launch B blocks, each block has U threads
b = cuda.blockIdx.x # // batch id
u = cuda.threadIdx.x # label id, u
T = xlen[b] # select AM length of current sample
U = ylen[b] + 1 # select target length of current sample, +1 for the blank token
labels: torch.Tensor = mlabels[
b
] # mb label start point, equivalent to mlabels + b * (maxU - 1)
offset = b * maxT * maxU # pointer indexing offset
# betas += offset # pointer offset, ignored since we explicitly add offset
# Initilize beta[b, t=T-1, u=U-1] for all b in B
# with log_probs[b, t=T-1, u=U-1, blank]
if u == 0:
betas[offset + (T - 1) * maxU + U - 1] = logp(
denom, acts, maxT, maxU, alphabet_size, b, T - 1, U - 1, blank_
)
# sync until all betas are initialized
cuda.syncthreads()
# Ordinary beta calculations, broadcast across B=b and U=u
# Look up backward variable calculation from rnnt_numpy.backward_pass()
for n in range(T + U - 2, -1, -1):
t = n - u
if u == (U - 1):
# for t in reversed(range(T - 1)) step to initialize betas[b, t, U-1]
if t >= 0 and t < (T - 1):
betas[offset + t * maxU + U - 1] = betas[
offset + (t + 1) * maxU + U - 1
] + logp(denom, acts, maxT, maxU, alphabet_size, b, t, U - 1, blank_)
elif u < U:
if t == T - 1:
# for u in reversed(range(U - 1)) step to initialize betas[b, T-1, u]
betas[offset + (T - 1) * maxU + u] = betas[
offset + (T - 1) * maxU + u + 1
] + logp(denom, acts, maxT, maxU, alphabet_size, b, T - 1, u, labels[u])
elif (t >= 0) and (t < T - 1):
# for t in reversed(range(T - 1)) for u in reversed(range(U - 1))
# step to compute betas[b, t, u]
no_emit = betas[offset + (t + 1) * maxU + u] + logp(
denom, acts, maxT, maxU, alphabet_size, b, t, u, blank_
)
emit = betas[offset + t * maxU + u + 1] + logp(
denom, acts, maxT, maxU, alphabet_size, b, t, u, labels[u]
)
betas[offset + t * maxU + u] = rnnt_helper.log_sum_exp(emit, no_emit)
# sync across all B=b and U=u
cuda.syncthreads()
# After final sync, betas[b, 0, 0] gives
# log-likelihood of backward pass.
if u == 0:
llBackward[b] = betas[offset]
[docs]@cuda.jit()
def compute_grad_kernel(
grads: torch.Tensor,
acts: torch.Tensor,
denom: torch.Tensor,
alphas: torch.Tensor,
betas: torch.Tensor,
logll: torch.Tensor,
xlen: torch.Tensor,
ylen: torch.Tensor,
mlabels: torch.Tensor, # [B, U]
minibatch: int,
maxT: int,
maxU: int,
alphabet_size: int,
blank_: int,
fastemit_lambda: float,
clamp: float,
):
"""
Compute gradients over the transduction step.
Args:
grads: Zero Tensor of shape [B, T, U, V+1]. Is updated by this kernel to
contain the gradients of this batch of samples.
acts: Tensor of shape [B, T, U, V+1] flattened.
Represents the logprobs activation tensor.
denom: Tensor of shape [B, T, U] flattened. Represents the denominator
of the logprobs activation tensor across entire vocabulary.
alphas: Alpha variable, contains forward probabilities.
A tensor of shape [B, T, U].
betas: Beta varoable, contains backward probabilities.
A tensor of shape [B, T, U].
logll: Log-likelihood of the forward variable, represented as a vector
of shape [B]. Represents the log-likelihood of the forward pass.
xlen: Vector of length B which contains the actual acoustic sequence
lengths in the padded activation tensor.
ylen: Vector of length B which contains the actual target sequence lengths
in the padded activation tensor.
mlabels: Matrix of shape [B, U+1] (+1 here is due to <SOS> token
- usually the RNNT blank). The matrix contains the padded target
transcription that must be predicted.
minibatch: Int representing the batch size.
maxT: The maximum possible acoustic sequence length.
Represents T in the logprobs tensor.
maxU: The maximum possible target sequence length.
Represents U in the logprobs tensor.
alphabet_size: The vocabulary dimension V+1 (inclusive of RNNT blank).
blank_: Index of the RNNT blank token in the vocabulary.
Generally the first or last token in the vocab.
fastemit_lambda: Float scaling factor for FastEmit regularization. Refer to
FastEmit: Low-latency Streaming ASR with Sequence-level
Emission Regularization.
clamp: Float value. When set to value >= 0.0, will clamp the
gradient to [-clamp, clamp].
Updates:
Kernel inplace updates the following inputs:
- grads: Gradients with respect to the log likelihood (logll).
"""
# Kernel call:
# blocks_per_grid = minibatch (b) * maxT (t) * maxU (u)
# threads_per_block = constant buffer size of parallel threads (v :: Constant)
tid = cuda.threadIdx.x # represents v, taking steps of some constant size
idx = tid # index of v < V+1; in steps of constant buffer size
col = cuda.blockIdx.x # represents a fused index of b * t * u
# Decompose original indices from fused `col`
u = col % maxU # (b * t * u) % u = u
bt = (col - u) // maxU # (b * t * u - u) // U = b * t
t = bt % maxT # (b * t) % t = t
mb = (bt - t) // maxT # (b * t - t) // T = b
# constants
T = xlen[mb] # select AM length of current sample
U = ylen[mb] + 1 # select target length of current sample, +1 for the blank token
labels: torch.Tensor = mlabels[mb] # labels = mlabels + mb * (maxU - 1);
# Buffered gradient calculations, broadcast across B=b, T=t and U=u,
# looped over V with some constant stride.
# Look up gradient calculation from rnnt_numpy.compute_gradient()
if t < T and u < U:
# For cuda kernels, maximum number of threads per block is limited to some value
# However, it may be the case that vocabulary size is larger than this limit
# To work around this, an arbitrary thread buffer size is chosen such that,
# 1) each element within the thread pool operates independently of the other
# 2) An inner while loop moves the index of each buffer element by the size
# of the buffer itself, such that all elements of the vocabulary size are
# covered in (V + 1 // thread_buffer) number of steps.
# As such, each thread will perform the while loop at least
# (V + 1 // thread_buffer) number of times
while idx < alphabet_size:
# remember, `col` represents the tri-index [b, t, u]
# therefore; logpk = denom[b, t, u] + acts[b, t, u, v]
logpk = denom[col] + acts[col * alphabet_size + idx]
# initialize the grad of the sample acts[b, t, u, v]
grad = math.exp(alphas[col] + betas[col] + logpk - logll[mb])
# If FastEmit regularization is enabled, calculate the gradeint of
# probability of predicting the next label at the current timestep.
# The formula for this is Equation 9 in https://arxiv.org/abs/2010.11148,
# multiplied by the log probability of the current step (t, u),
# normalized by the total log likelihood. Once the gradient has been
# calculated, scale it by `fastemit_lambda`, as in Equation 10.
if fastemit_lambda > 0.0 and u < U - 1:
fastemit_grad = fastemit_lambda * math.exp(
alphas[col] # alphas(t, u)
+ (
denom[col] + acts[col * alphabet_size + labels[u]]
) # y_hat(t, u)
+ betas[col + 1] # betas(t, u+1)
+ logpk # log Pr(k|t, u)
- logll[mb] # total log likelihood for normalization
)
else:
fastemit_grad = 0.0
# Update the gradient of act[b, t, u, v] with the gradient from
# FastEmit regularization
grad = grad + fastemit_grad
# // grad to last blank transition
# grad[b, T-1, U-1, v=blank] -= exp(alphas[b, t, u) + logpk - logll[b])
if (idx == blank_) and (t == T - 1) and (u == U - 1):
grad -= math.exp(alphas[col] + logpk - logll[mb])
# grad of blank across t < T;
# grad[b, t<T-1, u, v=blank] -= exp(alphas[b, t, u]
# + logpk - logll[b] betas[b, t + 1, u])
if (idx == blank_) and (t < T - 1):
grad -= math.exp(alphas[col] + logpk - logll[mb] + betas[col + maxU])
# grad of correct token across u < U;
# grad[b, t, u<U-1, v=label[u]] -= exp(alphas[b, t, u]
# + logpk - logll[b] + betas[b, t, u+1])
# Scale the gradient by (1.0 + FastEmit_lambda) in log space,
# then exponentiate
if (u < U - 1) and (idx == labels[u]):
# exp(log(1 + fastemit_lambda) + ...) is numerically more stable than
# multiplying (1.0 + fastemit_lambda) with result.
grad -= math.exp(
math.log1p(fastemit_lambda)
+ alphas[col]
+ logpk
- logll[mb]
+ betas[col + 1]
)
# update grads[b, t, u, v] = grad
grads[col * alphabet_size + idx] = grad
# clamp gradient (if needed)
if clamp > 0.0:
g = grads[col * alphabet_size + idx]
g = min(g, clamp)
g = max(g, -clamp)
grads[col * alphabet_size + idx] = g
# update internal index through the thread_buffer;
# until idx < V + 1, such that entire vocabulary has been updated.
idx += GPU_RNNT_THREAD_SIZE
[docs]@cuda.jit()
def compute_multiblank_alphas_kernel(
acts: torch.Tensor,
denom: torch.Tensor,
sigma: float,
alphas: torch.Tensor,
llForward: torch.Tensor,
xlen: torch.Tensor,
ylen: torch.Tensor,
mlabels: torch.Tensor,
minibatch: int,
maxT: int,
maxU: int,
alphabet_size: int,
blank_: int,
big_blank_duration: torch.Tensor,
num_big_blanks: int,
):
"""
Compute alpha (forward variable) probabilities for multi-blank transducuer loss
(https://arxiv.org/pdf/2211.03541).
Args:
acts: Tensor of shape [B, T, U, V + 1 + num_big_blanks] flattened.
Represents the logprobs activation tensor.
denom: Tensor of shape [B, T, U] flattened. Represents the denominator of
the logprobs activation tensor across entire vocabulary.
sigma: Hyper-parameter for logit-undernormalization technique for training
multi-blank transducers.
alphas: Zero tensor of shape [B, T, U]. Will be updated inside the kernel
with the forward variable probabilities.
llForward: Zero tensor of shape [B]. Represents the log-likelihood of the
forward pass. Returned as the forward pass loss that is
reduced by the optimizer.
xlen: Vector of length B which contains the actual acoustic sequence
lengths in the padded activation tensor.
ylen: Vector of length B which contains the actual target sequence
lengths in the padded activation tensor.
mlabels: Matrix of shape [B, U+1] (+1 here is due to <SOS> token
- usually the RNNT blank). The matrix contains the padded target
transcription that must be predicted.
minibatch: Int representing the batch size.
maxT: The maximum possible acoustic sequence length.
Represents T in the logprobs tensor.
maxU: The maximum possible target sequence length.
Represents U in the logprobs tensor.
alphabet_size: The vocabulary dimension V+1 (inclusive of RNNT blank).
blank_: Index of the RNNT standard blank token in the vocabulary.
big_blank_durations: Vector of supported big blank durations of the model.
num_big_blanks: Number of big blanks of the model.
Updates:
Kernel inplace updates the following inputs:
- alphas: forward variable scores.
- llForward: log-likelihood of forward variable.
"""
# // launch B blocks, each block has U threads
b = cuda.blockIdx.x # // batch id
u = cuda.threadIdx.x # label id, u
T = xlen[b] # select AM length of current sample
U = ylen[b] + 1 # select target length of current sample, +1 for the blank token
labels: torch.Tensor = mlabels[
b
] # mb label start point, equivalent to mlabels + b * (maxU - 1)
offset = b * maxT * maxU # pointer indexing offset
# Initilize alpha[b, t=0, u=0] for all b in B
if u == 0:
alphas[offset] = 0
# sync until all alphas are initialized
cuda.syncthreads()
# Ordinary alpha calculations, broadcast across B=b and U=u
# Look up forward variable calculation from rnnt_numpy.forward_pass()
# Note: because of the logit under-normalization, everytime logp() is called,
# it is always followed by a `-sigma` term.
for n in range(1, T + U - 1):
t = n - u
if u == 0:
# for t in range(1, T) step to initialize alphas[b, t, 0]
if t > 0 and t < T:
alphas[offset + t * maxU + u] = (
alphas[offset + (t - 1) * maxU + u]
+ logp(denom, acts, maxT, maxU, alphabet_size, b, t - 1, 0, blank_)
- sigma
)
# Now add the weights for big blanks.
for i in range(num_big_blanks):
if t >= big_blank_duration[i]:
alphas[offset + t * maxU + u] = rnnt_helper.log_sum_exp(
alphas[offset + t * maxU + u],
alphas[offset + (t - big_blank_duration[i]) * maxU + u]
+ logp(
denom,
acts,
maxT,
maxU,
alphabet_size,
b,
t - big_blank_duration[i],
0,
blank_ - 1 - i,
)
- sigma,
)
elif u < U:
# for u in range(1, U) step to initialize alphas[b, 0, u]
if t == 0:
alphas[offset + u] = (
alphas[offset + u - 1]
+ logp(
denom,
acts,
maxT,
maxU,
alphabet_size,
b,
0,
u - 1,
labels[u - 1],
)
- sigma
)
# for t in range(1, T) for u in range(1, U) step to compute alphas[b, t, u]
elif t > 0 and t < T:
no_emit = (
alphas[offset + (t - 1) * maxU + u]
+ logp(denom, acts, maxT, maxU, alphabet_size, b, t - 1, u, blank_)
- sigma
)
emit = (
alphas[offset + t * maxU + u - 1]
+ logp(
denom,
acts,
maxT,
maxU,
alphabet_size,
b,
t,
u - 1,
labels[u - 1],
)
- sigma
)
alphas[offset + t * maxU + u] = rnnt_helper.log_sum_exp(emit, no_emit)
# Now add the weights for big blanks.
for i in range(num_big_blanks):
if t >= big_blank_duration[i]:
# big-blank weight here is
# alpha(t - duration, u) * p(big-blank | t - duration, u)
# / exp(sigma), in log domain
# do this all all big-blanks if the above condition is met
big_blank_no_emit = (
alphas[offset + (t - big_blank_duration[i]) * maxU + u]
+ logp(
denom,
acts,
maxT,
maxU,
alphabet_size,
b,
t - big_blank_duration[i],
u,
blank_ - 1 - i,
)
- sigma
)
alphas[offset + t * maxU + u] = rnnt_helper.log_sum_exp(
alphas[offset + t * maxU + u], big_blank_no_emit
)
# sync across all B=b and U=u
cuda.syncthreads()
# After final sync, alphas[b, T-1, U - 1] + logprobs[b, T-1, U-1, blank]
# + denom[b, T-1, U-1] gives log-likelihood of forward pass.
if u == 0:
loglike = (
alphas[offset + (T - 1) * maxU + U - 1]
+ logp(denom, acts, maxT, maxU, alphabet_size, b, T - 1, U - 1, blank_)
- sigma
)
# Now add the weights for big blanks for the final weight computation.
for i in range(num_big_blanks):
if T >= big_blank_duration[i]:
big_blank_loglike = (
alphas[offset + (T - big_blank_duration[i]) * maxU + U - 1]
+ logp(
denom,
acts,
maxT,
maxU,
alphabet_size,
b,
T - big_blank_duration[i],
U - 1,
blank_ - 1 - i,
)
- sigma
)
loglike = rnnt_helper.log_sum_exp(loglike, big_blank_loglike)
llForward[b] = loglike
[docs]@cuda.jit()
def compute_multiblank_betas_kernel(
acts: torch.Tensor,
denom: torch.Tensor,
sigma: float,
betas: torch.Tensor,
llBackward: torch.Tensor,
xlen: torch.Tensor,
ylen: torch.Tensor,
mlabels: torch.Tensor, # [B, U]
minibatch: int,
maxT: int,
maxU: int,
alphabet_size: int,
blank_: int,
big_blank_duration: torch.Tensor,
num_big_blanks: int,
):
"""
Compute beta (backward variable) probabilities for multi-blank transducer loss
(https://arxiv.org/pdf/2211.03541).
Args:
acts: Tensor of shape [B, T, U, V + 1 + num-big-blanks] flattened.
Represents the logprobs activation tensor.
denom: Tensor of shape [B, T, U] flattened. Represents the denominator
of the logprobs activation tensor across entire vocabulary.
sigma: Hyper-parameter for logit-undernormalization technique for
training multi-blank transducers.
betas: Zero tensor of shape [B, T, U]. Will be updated inside the kernel
with the backward variable probabilities.
llBackward: Zero tensor of shape [B]. Represents the log-likelihood
of the backward pass. Returned as the backward pass loss
that is reduced by the optimizer.
xlen: Vector of length B which contains the actual acoustic sequence
lengths in the padded activation tensor.
ylen: Vector of length B which contains the actual target sequence
lengths in the padded activation tensor.
mlabels: Matrix of shape [B, U+1] (+1 here is due to <SOS> token
- usually the RNNT blank). The matrix contains the padded target
transcription that must be predicted.
minibatch: Int representing the batch size.
maxT: The maximum possible acoustic sequence length.
Represents T in the logprobs tensor.
maxU: The maximum possible target sequence length.
Represents U in the logprobs tensor.
alphabet_size: The vocabulary dimension V+1 (inclusive of RNNT blank).
blank_: Index of the RNNT standard blank token in the vocabulary.
big_blank_durations: Vector of supported big blank durations of the model.
num_big_blanks: Number of big blanks of the model.
Updates:
Kernel inplace updates the following inputs:
- betas: backward variable scores.
- llBackward: log-likelihood of backward variable.
"""
# // launch B blocks, each block has U threads
b = cuda.blockIdx.x # // batch id
u = cuda.threadIdx.x # label id, u
T = xlen[b] # select AM length of current sample
U = ylen[b] + 1 # select target length of current sample, +1 for the blank token
labels: torch.Tensor = mlabels[
b
] # mb label start point, equivalent to mlabels + b * (maxU - 1)
offset = b * maxT * maxU # pointer indexing offset
# Note: just like the alphas, because of the logit under-normalization, everytime
# logp() is called, it is always followed by a `-sigma` term.
# Initilize beta[b, t=T-1, u=U-1] for all b in B with
# log_probs[b, t=T-1, u=U-1, blank]
if u == 0:
betas[offset + (T - 1) * maxU + U - 1] = (
logp(denom, acts, maxT, maxU, alphabet_size, b, T - 1, U - 1, blank_)
- sigma
)
# sync until all betas are initialized
cuda.syncthreads()
# Ordinary beta calculations, broadcast across B=b and U=u
# Look up backward variable calculation from rnnt_numpy.backward_pass()
for n in range(T + U - 2, -1, -1):
t = n - u
if u == (U - 1):
# for t in reversed(range(T - 1)) step to initialize betas[b, t, U-1]
if t >= 0 and t < (T - 1):
# beta[t, U - 1] = beta[t + 1, U - 1] * p(blank | t, U - 1) / exp(sigma)
# this part is the same as regular RNN-T.
betas[offset + t * maxU + U - 1] = (
betas[offset + (t + 1) * maxU + U - 1]
+ logp(denom, acts, maxT, maxU, alphabet_size, b, t, U - 1, blank_)
- sigma
)
# now add the weights from big blanks
for i in range(num_big_blanks):
if t + big_blank_duration[i] < T:
# adding to beta[t, U - 1] of weight (in log domain),
# beta[t + duration, U - 1] *
# p(big-blank | t, U - 1) / exp(sigma)
betas[offset + t * maxU + U - 1] = rnnt_helper.log_sum_exp(
betas[offset + t * maxU + U - 1],
betas[offset + (t + big_blank_duration[i]) * maxU + U - 1]
+ logp(
denom,
acts,
maxT,
maxU,
alphabet_size,
b,
t,
U - 1,
blank_ - 1 - i,
)
- sigma,
)
elif t + big_blank_duration[i] == T and big_blank_duration[i] != 1:
# adding to beta[T - duration, U - 1] of weight (in log domain),
# p(big-blank | T - duration, U - 1) / exp(sigma)
betas[offset + t * maxU + U - 1] = rnnt_helper.log_sum_exp(
betas[offset + t * maxU + U - 1],
logp(
denom,
acts,
maxT,
maxU,
alphabet_size,
b,
t,
U - 1,
blank_ - 1 - i,
)
- sigma,
)
elif u < U:
if t == T - 1:
# for u in reversed(range(U - 1)) step to initialize betas[b, T-1, u]
betas[offset + (T - 1) * maxU + u] = (
betas[offset + (T - 1) * maxU + u + 1]
+ logp(
denom, acts, maxT, maxU, alphabet_size, b, T - 1, u, labels[u]
)
- sigma
)
elif (t >= 0) and (t < T - 1):
# for t in reversed(range(T - 1)) for u in reversed(range(U - 1))
# step to compute betas[b, t, u]
no_emit = (
betas[offset + (t + 1) * maxU + u]
+ logp(denom, acts, maxT, maxU, alphabet_size, b, t, u, blank_)
- sigma
)
emit = (
betas[offset + t * maxU + u + 1]
+ logp(denom, acts, maxT, maxU, alphabet_size, b, t, u, labels[u])
- sigma
)
betas[offset + t * maxU + u] = rnnt_helper.log_sum_exp(emit, no_emit)
# now add the weights from big blanks
for i in range(num_big_blanks):
if t < T - big_blank_duration[i]:
# added weight for the big-blank,
# beta[t + duration, u] * p(big-blank | t, u) / exp(sigma)
big_blank_no_emit = (
betas[offset + (t + big_blank_duration[i]) * maxU + u]
+ logp(
denom,
acts,
maxT,
maxU,
alphabet_size,
b,
t,
u,
blank_ - 1 - i,
)
- sigma
)
betas[offset + t * maxU + u] = rnnt_helper.log_sum_exp(
betas[offset + t * maxU + u], big_blank_no_emit
)
# sync across all B=b and U=u
cuda.syncthreads()
# After final sync, betas[b, 0, 0] gives
# log-likelihood of backward pass.
if u == 0:
llBackward[b] = betas[offset]
[docs]@cuda.jit()
def compute_multiblank_grad_kernel(
grads: torch.Tensor,
acts: torch.Tensor,
denom: torch.Tensor,
sigma: float,
alphas: torch.Tensor,
betas: torch.Tensor,
logll: torch.Tensor,
xlen: torch.Tensor,
ylen: torch.Tensor,
mlabels: torch.Tensor, # [B, U]
minibatch: int,
maxT: int,
maxU: int,
alphabet_size: int,
blank_: int,
big_blank_duration: torch.Tensor,
num_big_blanks: int,
fastemit_lambda: float,
clamp: float,
):
"""
Compute gradients for multi-blank transducer loss
(https://arxiv.org/pdf/2211.03541).
Args:
grads: Zero Tensor of shape [B, T, U, V + 1 + num_big_blanks].
Is updated by this kernel to contain the gradients of this batch of samples.
acts: Tensor of shape [B, T, U, V + 1 + num_big_blanks] flattened.
Represents the logprobs activation tensor.
denom: Tensor of shape [B, T, U] flattened. Represents the denominator
of the logprobs activation tensor across entire vocabulary.
sigma: Hyper-parameter for logit-undernormalization technique
for training multi-blank transducers.
alphas: Alpha variable, contains forward probabilities.
A tensor of shape [B, T, U].
betas: Beta varoable, contains backward probabilities.
A tensor of shape [B, T, U].
logll: Log-likelihood of the forward variable, represented as
a vector of shape [B]. Represents the log-likelihood of the forward pass.
xlen: Vector of length B which contains the actual acoustic
sequence lengths in the padded activation tensor.
ylen: Vector of length B which contains the actual target sequence
lengths in the padded activation tensor.
mlabels: Matrix of shape [B, U+1] (+1 here is due to <SOS> token
- usually the RNNT blank). The matrix contains the padded target
transcription that must be predicted.
minibatch: Int representing the batch size.
maxT: The maximum possible acoustic sequence length.
Represents T in the logprobs tensor.
maxU: The maximum possible target sequence length.
Represents U in the logprobs tensor.
alphabet_size: The vocabulary dimension V+1 (inclusive of RNNT blank).
blank_: Index of the RNNT blank token in the vocabulary.
Generally the first or last token in the vocab.
fastemit_lambda: Float scaling factor for FastEmit regularization. Refer to
FastEmit: Low-latency Streaming ASR with Sequence-level
Emission Regularization.
clamp: Float value. When set to value >= 0.0, will clamp
the gradient to [-clamp, clamp].
big_blank_durations: Vector of supported big blank durations of the model.
num_big_blanks: Number of big blanks of the model.
Updates:
Kernel inplace updates the following inputs:
- grads: Gradients with respect to the log likelihood (logll).
"""
# Kernel call:
# blocks_per_grid = minibatch (b) * maxT (t) * maxU (u)
# threads_per_block = constant buffer size of parallel threads (v :: Constant)
tid = cuda.threadIdx.x # represents v, taking steps of some constant size
idx = tid # index of v < V+1; in steps of constant buffer size
col = cuda.blockIdx.x # represents a fused index of b * t * u
# Decompose original indices from fused `col`
u = col % maxU # (b * t * u) % u = u
bt = (col - u) // maxU # (b * t * u - u) // U = b * t
t = bt % maxT # (b * t) % t = t
mb = (bt - t) // maxT # (b * t - t) // T = b
# constants
T = xlen[mb] # select AM length of current sample
U = ylen[mb] + 1 # select target length of current sample, +1 for the blank token
labels: torch.Tensor = mlabels[mb] # labels = mlabels + mb * (maxU - 1);
# Buffered gradient calculations, broadcast across B=b, T=t and U=u, looped over
# V with some constant stride. Look up gradient calculation from
# rnnt_numpy.compute_gradient()
if t < T and u < U:
# For cuda kernels, maximum number of threads per block is limited to some value
# However, it may be the case that vocabulary size is larger than this limit
# To work around this, an arbitrary thread buffer size is chosen such that,
# 1) each element within the thread pool operates independently of the other
# 2) An inner while loop moves the index of each buffer element by the size
# of the buffer itself, such that all elements of the vocabulary size are
# covered in (V + 1 // thread_buffer) number of steps.
# As such, each thread will perform the while loop at least
# (V + 1 // thread_buffer) number of times
while idx < alphabet_size:
# remember, `col` represents the tri-index [b, t, u]
# therefore; logpk = denom[b, t, u] + acts[b, t, u, v]
logpk = denom[col] + acts[col * alphabet_size + idx]
# initialize the grad of the sample acts[b, t, u, v]
grad = math.exp(alphas[col] + betas[col] + logpk - logll[mb])
# In all of the following computation, whenever logpk is used, we
# need to subtract sigma based on our derivation of the gradient of
# the logit under-normalization method.
# If FastEmit regularization is enabled, calculate the gradeint of
# probability of predicting the next label at the current timestep.
# The formula for this is Equation 9 in https://arxiv.org/abs/2010.11148,
# multiplied by the log probability of the current step (t, u), normalized
# by the total log likelihood. Once the gradient has been calculated,
# scale it by `fastemit_lambda`, as in Equation 10.
if fastemit_lambda > 0.0 and u < U - 1:
fastemit_grad = fastemit_lambda * math.exp(
alphas[col] # alphas(t, u)
+ (denom[col] + acts[col * alphabet_size + labels[u]])
+ betas[col + 1] # betas(t, u+1)
+ logpk # log Pr(k|t, u)
- sigma
- logll[mb] # total log likelihood for normalization
)
else:
fastemit_grad = 0.0
# Update the gradient of act[b, t, u, v] with the gradient
# from FastEmit regularization
grad = grad + fastemit_grad
# grad to last blank transition
# grad[b, T-1, U-1, v=blank] -= exp(alphas[b, t, u)
# + logpk - sigma - logll[b])
if (idx == blank_) and (t == T - 1) and (u == U - 1):
grad -= math.exp(alphas[col] + logpk - sigma - logll[mb])
else:
# this is one difference of the multi-blank gradient from standard RNN-T
# gradient -- basically, wherever the blank_ symbol is addressed in the
# original code, we need to do similar things to big blanks, and we need
# to change the if conditions to match the duration of the big-blank.
# grad[b, T-duration, U-1, v=big-blank] -=
# exp(alphas[b, t, u) + logpk - sigma - logll[b])
for i in range(num_big_blanks):
if (
(idx == blank_ - 1 - i)
and (t == T - big_blank_duration[i])
and (u == U - 1)
):
grad -= math.exp(alphas[col] + logpk - sigma - logll[mb])
# grad of blank across t < T;
# grad[b, t<T-1, u, v=blank] -= exp(alphas[b, t, u] +
# logpk - sigma - logll[b] betas[b, t + 1, u])
if (idx == blank_) and (t < T - 1):
grad -= math.exp(
alphas[col] + logpk - sigma - logll[mb] + betas[col + maxU]
)
else:
# This is another difference between multi-blank and RNN-T gradients.
# Now we consider gradients for big-blanks.
# grad[b, t<T-duration, u, v=big-blank] -=
# exp(alphas[b, t, u] + logpk - sigma - logll[b]
# + betas[b, t + duration, u])
for i in range(num_big_blanks):
if (idx == blank_ - 1 - i) and (t < T - big_blank_duration[i]):
grad -= math.exp(
alphas[col]
+ logpk
- sigma
- logll[mb]
+ betas[col + big_blank_duration[i] * maxU]
)
# grad of correct token across u < U;
# grad[b, t, u<U-1, v=label[u]] -=
# exp(alphas[b, t, u] + logpk - sigma - logll[b] + betas[b, t, u+1])
# Scale the gradient by (1.0 + FastEmit_lambda) in log space,
# then exponentiate
if (u < U - 1) and (idx == labels[u]):
# exp(log(1 + fastemit_lambda) + ...) is numerically more stable than
# multiplying (1.0 + fastemit_lambda) with result.
grad -= math.exp(
math.log1p(fastemit_lambda)
+ alphas[col]
+ logpk
- sigma
- logll[mb]
+ betas[col + 1]
)
# update grads[b, t, u, v] = grad
grads[col * alphabet_size + idx] = grad
# clamp gradient (if needed)
if clamp > 0.0:
g = grads[col * alphabet_size + idx]
g = min(g, clamp)
g = max(g, -clamp)
grads[col * alphabet_size + idx] = g
# update internal index through the thread_buffer;
# until idx < V + 1, such that entire vocabulary has been updated.
idx += GPU_RNNT_THREAD_SIZE
```