model.py

# MIT License
# 
# Copyright (c) 2022 Tada Makepeace
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"""This module contains a modern version of the DeepSpeech2 speech recognition
model which comes from from Michael Nguyen, Machine Learning Research Engineer
at AssemblyAI and Niko Laskaris at Comet.ml"""

import torch.nn as nn
import torch.nn.functional as F


class CNNLayerNorm(nn.Module):
    """Layer normalization built for cnns input"""
    def __init__(self, n_feats):
        super(CNNLayerNorm, self).__init__()
        self.layer_norm = nn.LayerNorm(n_feats)

    def forward(self, x):
        # x (batch, channel, feature, time)
        x = x.transpose(2, 3).contiguous() # (batch, channel, time, feature)
        x = self.layer_norm(x)
        return x.transpose(2, 3).contiguous() # (batch, channel, feature, time) 


class ResidualCNN(nn.Module):
    """Residual CNN inspired by https://arxiv.org/pdf/1603.05027.pdf
        except with layer norm instead of batch norm
    """
    def __init__(self, in_channels, out_channels, kernel, stride, dropout, n_feats):
        super(ResidualCNN, self).__init__()

        self.cnn1 = nn.Conv2d(in_channels, out_channels, kernel, stride, padding=kernel//2)
        self.cnn2 = nn.Conv2d(out_channels, out_channels, kernel, stride, padding=kernel//2)
        self.dropout1 = nn.Dropout(dropout)
        self.dropout2 = nn.Dropout(dropout)
        self.layer_norm1 = CNNLayerNorm(n_feats)
        self.layer_norm2 = CNNLayerNorm(n_feats)

    def forward(self, x):
        residual = x  # (batch, channel, feature, time)
        x = self.layer_norm1(x)
        x = F.gelu(x)
        x = self.dropout1(x)
        x = self.cnn1(x)
        x = self.layer_norm2(x)
        x = F.gelu(x)
        x = self.dropout2(x)
        x = self.cnn2(x)
        x += residual
        return x # (batch, channel, feature, time)


class BidirectionalGRU(nn.Module):
    def __init__(self, rnn_dim, hidden_size, dropout, batch_first):
        super(BidirectionalGRU, self).__init__()

        self.BiGRU = nn.GRU(
            input_size=rnn_dim, hidden_size=hidden_size,
            num_layers=1, batch_first=batch_first, bidirectional=True)
        self.layer_norm = nn.LayerNorm(rnn_dim)
        self.dropout = nn.Dropout(dropout)

    def forward(self, x):
        x = self.layer_norm(x)
        x = F.gelu(x)
        x, _ = self.BiGRU(x)
        x = self.dropout(x)
        return x


class SpeechRecognitionModel(nn.Module):
    """Modern version of the DeepSpeech2 from Michael Nguyen, Machine
    Learning Research Engineer at AssemblyAI and Niko Laskaris at Comet.ml"""
    def __init__(self, n_cnn_layers, n_rnn_layers, rnn_dim, n_class, n_feats, stride=2, dropout=0.1):
        super(SpeechRecognitionModel, self).__init__()
        n_feats = n_feats//2
        self.cnn = nn.Conv2d(1, 32, 3, stride=stride, padding=3//2)  # cnn for extracting heirachal features

        # n residual cnn layers with filter size of 32
        self.rescnn_layers = nn.Sequential(*[
            ResidualCNN(32, 32, kernel=3, stride=1, dropout=dropout, n_feats=n_feats) 
            for _ in range(n_cnn_layers)
        ])
        self.fully_connected = nn.Linear(n_feats*32, rnn_dim)
        self.birnn_layers = nn.Sequential(*[
            BidirectionalGRU(rnn_dim=rnn_dim if i==0 else rnn_dim*2,
                             hidden_size=rnn_dim, dropout=dropout, batch_first=i==0)
            for i in range(n_rnn_layers)
        ])
        self.classifier = nn.Sequential(
            nn.Linear(rnn_dim*2, rnn_dim),  # birnn returns rnn_dim*2
            nn.GELU(),
            nn.Dropout(dropout),
            nn.Linear(rnn_dim, n_class)
        )

    def forward(self, x):
        x = self.cnn(x)
        x = self.rescnn_layers(x)
        sizes = x.size()
        x = x.view(sizes[0], sizes[1] * sizes[2], sizes[3])  # (batch, feature, time)
        x = x.transpose(1, 2) # (batch, time, feature)
        x = self.fully_connected(x)
        x = self.birnn_layers(x)
        x = self.classifier(x)
        return x