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# Copyright 2021 Xiaomi Corp. (authors: Fangjun Kuang)
#
# See ../../../../LICENSE for clarification regarding multiple authors
#
# 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 torch
import torch.nn as nn
import torch.nn.functional as F
from scaling import ScaledConv1d, ScaledEmbedding
class Decoder(nn.Module):
"""This class modifies the stateless decoder from the following paper:
RNN-transducer with stateless prediction network
https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=9054419
It removes the recurrent connection from the decoder, i.e., the prediction
network. Different from the above paper, it adds an extra Conv1d
right after the embedding layer.
TODO: Implement https://arxiv.org/pdf/2109.07513.pdf
"""
def __init__(
self,
vocab_size: int,
decoder_dim: int,
blank_id: int,
context_size: int,
):
"""
Args:
vocab_size:
Number of tokens of the modeling unit including blank.
decoder_dim:
Dimension of the input embedding, and of the decoder output.
blank_id:
The ID of the blank symbol.
context_size:
Number of previous words to use to predict the next word.
1 means bigram; 2 means trigram. n means (n+1)-gram.
"""
super().__init__()
self.embedding = ScaledEmbedding(
num_embeddings=vocab_size,
embedding_dim=decoder_dim,
padding_idx=blank_id,
)
self.blank_id = blank_id
assert context_size >= 1, context_size
self.context_size = context_size
self.vocab_size = vocab_size
if context_size > 1:
self.conv = ScaledConv1d(
in_channels=decoder_dim,
out_channels=decoder_dim,
kernel_size=context_size,
padding=0,
groups=decoder_dim,
bias=False,
)
def forward(self, y: torch.Tensor, need_pad: bool = True) -> torch.Tensor:
"""
Args:
y:
A 2-D tensor of shape (N, U).
need_pad:
True to left pad the input. Should be True during training.
False to not pad the input. Should be False during inference.
Returns:
Return a tensor of shape (N, U, decoder_dim).
"""
y = y.to(torch.int64)
embedding_out = self.embedding(y)
if self.context_size > 1:
embedding_out = embedding_out.permute(0, 2, 1)
if need_pad is True:
embedding_out = F.pad(embedding_out, pad=(self.context_size - 1, 0))
else:
# During inference time, there is no need to do extra padding
# as we only need one output
assert embedding_out.size(-1) == self.context_size
embedding_out = self.conv(embedding_out)
embedding_out = embedding_out.permute(0, 2, 1)
embedding_out = F.relu(embedding_out)
return embedding_out
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