decoder.py

import itertools

import torch
import torch.nn as nn

import seq2seq.data.config as config
from seq2seq.models.attention import BahdanauAttention
from seq2seq.utils import init_lstm_


class RecurrentAttention(nn.Module):
    """
    LSTM wrapped with an attention module.
    """
    def __init__(self, input_size=1024, context_size=1024, hidden_size=1024,
                 num_layers=1, batch_first=False, dropout=0.2,
                 init_weight=0.1, fusion=True):
        """
        Constructor for the RecurrentAttention.

        :param input_size: number of features in input tensor
        :param context_size: number of features in output from encoder
        :param hidden_size: internal hidden size
        :param num_layers: number of layers in LSTM
        :param batch_first: if True the model uses (batch,seq,feature) tensors,
            if false the model uses (seq, batch, feature)
        :param dropout: probability of dropout (on input to LSTM layer)
        :param init_weight: range for the uniform initializer
        """

        super(RecurrentAttention, self).__init__()

        self.rnn = nn.LSTM(input_size, hidden_size, num_layers, bias=True,
                           batch_first=batch_first)
        init_lstm_(self.rnn, init_weight)

        self.attn = BahdanauAttention(hidden_size, context_size, context_size,
                                      normalize=True, batch_first=batch_first,
                                      fusion=fusion)

        self.dropout = nn.Dropout(dropout)

    def forward(self, inputs, hidden, context, context_len):
        """
        Execute RecurrentAttention.

        :param inputs: tensor with inputs
        :param hidden: hidden state for LSTM layer
        :param context: context tensor from encoder
        :param context_len: vector of encoder sequence lengths

        :returns (rnn_outputs, hidden, attn_output, attn_scores)
        """
        # set attention mask, sequences have different lengths, this mask
        # allows to include only valid elements of context in attention's
        # softmax
        self.attn.set_mask(context_len, context)

        inputs = self.dropout(inputs)
        rnn_outputs, hidden = self.rnn(inputs, hidden)
        attn_outputs, scores = self.attn(rnn_outputs, context)

        return rnn_outputs, hidden, attn_outputs, scores


class Classifier(nn.Module):
    """
    Fully-connected classifier
    """
    def __init__(self, in_features, out_features, init_weight=0.1):
        """
        Constructor for the Classifier.

        :param in_features: number of input features
        :param out_features: number of output features (size of vocabulary)
        :param init_weight: range for the uniform initializer
        """
        super(Classifier, self).__init__()
        self.classifier = nn.Linear(in_features, out_features)
        nn.init.uniform_(self.classifier.weight.data, -init_weight, init_weight)
        nn.init.uniform_(self.classifier.bias.data, -init_weight, init_weight)

    def forward(self, x):
        """
        Execute the classifier.

        :param x: output from decoder
        """
        out = self.classifier(x)
        return out


class ResidualRecurrentDecoder(nn.Module):
    """
    Decoder with Embedding, LSTM layers, attention, residual connections and
    optinal dropout.

    Attention implemented in this module is different than the attention
    discussed in the GNMT arxiv paper. In this model the output from the first
    LSTM layer of the decoder goes into the attention module, then the
    re-weighted context is concatenated with inputs to all subsequent LSTM
    layers in the decoder at the current timestep.

    Residual connections are enabled after 3rd LSTM layer, dropout is applied
    on inputs to LSTM layers.
    """
    def __init__(self, vocab_size, hidden_size=1024, num_layers=4, dropout=0.2,
                 batch_first=False, embedder=None, init_weight=0.1, fusion=True):
        """
        Constructor of the ResidualRecurrentDecoder.

        :param vocab_size: size of vocabulary
        :param hidden_size: hidden size for LSMT layers
        :param num_layers: number of LSTM layers
        :param dropout: probability of dropout (on input to LSTM layers)
        :param batch_first: if True the model uses (batch,seq,feature) tensors,
            if false the model uses (seq, batch, feature)
        :param embedder: instance of nn.Embedding, if None constructor will
            create new embedding layer
        :param init_weight: range for the uniform initializer
        """
        super(ResidualRecurrentDecoder, self).__init__()

        self.num_layers = num_layers

        self.att_rnn = RecurrentAttention(hidden_size, hidden_size,
                                          hidden_size, num_layers=1,
                                          batch_first=batch_first,
                                          dropout=dropout,
                                          fusion=fusion)

        self.rnn_layers = nn.ModuleList()
        for _ in range(num_layers - 1):
            self.rnn_layers.append(
                nn.LSTM(2 * hidden_size, hidden_size, num_layers=1, bias=True,
                        batch_first=batch_first))

        for lstm in self.rnn_layers:
            init_lstm_(lstm, init_weight)

        self.share_embedding = (embedder is not None)
        if embedder is not None:
            self.embedder = embedder
        else:
            self.embedder = nn.Embedding(vocab_size, hidden_size,
                                         padding_idx=config.PAD)
            nn.init.uniform_(self.embedder.weight.data, -init_weight, init_weight)

        self.classifier = Classifier(hidden_size, vocab_size)
        self.dropout = nn.Dropout(p=dropout)

    def init_hidden(self, hidden):
        """
        Converts flattened hidden state (from sequence generator) into a tuple
        of hidden states.

        :param hidden: None or flattened hidden state for decoder RNN layers
        """
        if hidden is not None:
            # per-layer chunks
            hidden = hidden.chunk(self.num_layers)
            # (h, c) chunks for LSTM layer
            hidden = tuple(i.chunk(2) for i in hidden)
        else:
            hidden = [None] * self.num_layers

        self.next_hidden = []
        return hidden

    def append_hidden(self, h):
        """
        Appends the hidden vector h to the list of internal hidden states.

        :param h: hidden vector
        """
        if self.inference:
            self.next_hidden.append(h)

    def package_hidden(self):
        """
        Flattens the hidden state from all LSTM layers into one tensor (for
        the sequence generator).
        """
        if self.inference:
            hidden = torch.cat(tuple(itertools.chain(*self.next_hidden)))
        else:
            hidden = None
        return hidden

    def forward(self, inputs, context, inference=False):
        """
        Execute the decoder.

        :param inputs: tensor with inputs to the decoder
        :param context: state of encoder, encoder sequence lengths and hidden
            state of decoder's LSTM layers
        :param inference: if True stores and repackages hidden state
        """
        self.inference = inference

        enc_context, enc_len, hidden = context
        hidden = self.init_hidden(hidden)

        if self.share_embedding and self.training:
            x = inputs
        else:
            x = self.embedder(inputs)

        x, h, attn, scores = self.att_rnn(x, hidden[0], enc_context, enc_len)
        self.append_hidden(h)

        x = torch.cat((x, attn), dim=2)
        x = self.dropout(x)
        x, h = self.rnn_layers[0](x, hidden[1])
        self.append_hidden(h)

        for i in range(1, len(self.rnn_layers)):
            residual = x
            x = torch.cat((x, attn), dim=2)
            x = self.dropout(x)
            x, h = self.rnn_layers[i](x, hidden[i + 1])
            self.append_hidden(h)
            x = x + residual

        x = self.classifier(x)
        hidden = self.package_hidden()

        return x, scores, [enc_context, enc_len, hidden]