Simple batched PyTorch LSTM

Pytorch_LSTM_variable_mini_batches.py

import torch
import torch.nn as nn
from torch.autograd import Variable
from torch.nn import functional as F

"""
Blog post:
Taming LSTMs: Variable-sized mini-batches and why PyTorch is good for your health:
https://medium.com/@_willfalcon/taming-lstms-variable-sized-mini-batches-and-why-pytorch-is-good-for-your-health-61d35642972e
"""


class BieberLSTM(nn.Module):
    def __init__(self, nb_layers, nb_lstm_units=100, embedding_dim=3, batch_size=3):
        self.vocab = {'<PAD>': 0, 'is': 1, 'it': 2, 'too': 3, 'late': 4, 'now': 5, 'say': 6, 'sorry': 7, 'ooh': 8,
                      'yeah': 9}
        self.tags = {'<PAD>': 0, 'VB': 1, 'PRP': 2, 'RB': 3, 'JJ': 4, 'NNP': 5}

        self.nb_layers = nb_layers
        self.nb_lstm_units = nb_lstm_units
        self.embedding_dim = embedding_dim
        self.batch_size = batch_size

        # don't count the padding tag for the classifier output
        self.nb_tags = len(self.tags) - 1

        # when the model is bidirectional we double the output dimension
        self.lstm

        # build actual NN
        self.__build_model()

    def __build_model(self):
        # build embedding layer first
        nb_vocab_words = len(self.vocab)

        # whenever the embedding sees the padding index it'll make the whole vector zeros
        padding_idx = self.vocab['<PAD>']
        self.word_embedding = nn.Embedding(
            num_embeddings=nb_vocab_words,
            embedding_dim=self.embedding_dim,
            padding_idx=padding_idx
        )

        # design LSTM
        self.lstm = nn.LSTM(
            input_size=self.embedding_dim,
            hidden_size=self.nb_lstm_units,
            num_layers=self.nb_lstm_layers,
            batch_first=True,
        )

        # output layer which projects back to tag space
        self.hidden_to_tag = nn.Linear(self.nb_lstm_units, self.nb_tags)

    def init_hidden(self):
        # the weights are of the form (nb_layers, batch_size, nb_lstm_units)
        hidden_a = torch.randn(self.hparams.nb_lstm_layers, self.batch_size, self.nb_lstm_units)
        hidden_b = torch.randn(self.hparams.nb_lstm_layers, self.batch_size, self.nb_lstm_units)

        if self.hparams.on_gpu:
            hidden_a = hidden_a.cuda()
            hidden_b = hidden_b.cuda()

        hidden_a = Variable(hidden_a)
        hidden_b = Variable(hidden_b)

        return (hidden_a, hidden_b)

    def forward(self, X, X_lengths):
        # reset the LSTM hidden state. Must be done before you run a new batch. Otherwise the LSTM will treat
        # a new batch as a continuation of a sequence
        self.hidden = self.init_hidden()

        batch_size, seq_len, _ = X.size()

        # ---------------------
        # 1. embed the input
        # Dim transformation: (batch_size, seq_len, 1) -> (batch_size, seq_len, embedding_dim)
        X = self.word_embedding(X)

        # ---------------------
        # 2. Run through RNN
        # TRICK 2 ********************************
        # Dim transformation: (batch_size, seq_len, embedding_dim) -> (batch_size, seq_len, nb_lstm_units)

        # pack_padded_sequence so that padded items in the sequence won't be shown to the LSTM
        X = torch.nn.utils.rnn.pack_padded_sequence(x, X_lengths, batch_first=True)

        # now run through LSTM
        X, self.hidden = self.lstm(X, self.hidden)

        # undo the packing operation
        X, _ = torch.nn.utils.rnn.pad_packed_sequence(X, batch_first=True)

        # ---------------------
        # 3. Project to tag space
        # Dim transformation: (batch_size, seq_len, nb_lstm_units) -> (batch_size * seq_len, nb_lstm_units)

        # this one is a bit tricky as well. First we need to reshape the data so it goes into the linear layer
        X = X.contiguous()
        X = X.view(-1, X.shape[2])

        # run through actual linear layer
        X = self.hidden_to_tag(X)

        # ---------------------
        # 4. Create softmax activations bc we're doing classification
        # Dim transformation: (batch_size * seq_len, nb_lstm_units) -> (batch_size, seq_len, nb_tags)
        X = F.log_softmax(X, dim=1)

        # I like to reshape for mental sanity so we're back to (batch_size, seq_len, nb_tags)
        X = X.view(batch_size, seq_len, self.nb_tags)

        Y_hat = X
        return Y_hat

    def loss(self, Y_hat, Y, X_lengths):
        # TRICK 3 ********************************
        # before we calculate the negative log likelihood, we need to mask out the activations
        # this means we don't want to take into account padded items in the output vector
        # simplest way to think about this is to flatten ALL sequences into a REALLY long sequence
        # and calculate the loss on that.

        # flatten all the labels
        Y = Y.view(-1)

        # flatten all predictions
        Y_hat = Y_hat.view(-1, self.nb_tags)

        # create a mask by filtering out all tokens that ARE NOT the padding token
        tag_pad_token = self.tags['<PAD>']
        mask = (Y > tag_pad_token).float()

        # count how many tokens we have
        nb_tokens = int(torch.sum(mask).data[0])

        # pick the values for the label and zero out the rest with the mask
        Y_hat = Y_hat[range(Y_hat.shape[0]), Y] * mask

        # compute cross entropy loss which ignores all <PAD> tokens
        ce_loss = -torch.sum(Y_hat) / nb_tokens

        return ce_loss

Thanks for the tutorial! In addition to the above comments which I agree with, could you also add super(BieberLSTM, self).__init__() in def __init__()? Without this, I got an error AttributeError: cannot assign module before Module.__init__() call. See the discussion here: https://discuss.pytorch.org/t/attributeerror-cannot-assign-module-before-module---init---call/1446/1

I have a question,

why should the hidden states be initialized randomly?

        hidden_a = torch.randn(self.hparams.nb_lstm_layers, self.batch_size, self.nb_lstm_units)
        hidden_b = torch.randn(self.hparams.nb_lstm_layers, self.batch_size, self.nb_lstm_units)

most of the examples I saw using zeros, which makes more sense to me?

Thank you for the awesome tutorial on NLP embedding

This is a bug in line 128 with respect to the shaping before calculating the cross-entropy loss

# pick the values for the label and zero out the rest with the mask
Y_hat = Y_hat[range(Y_hat.shape[0]), Y] * mask

Here Y and mask have the same shape (batch_size * seq_len * nb_tags) after flattening it. We want to let Y_hat has the same shape so that we could adopt the dot product and calculate the cross-entropy.

Thus, it should be Y_hat = Y_hat.view_as(Y) * mask

Hope I understood your comment. Correct me if I happen to be wrong. Thanks in advance.

---

Can someone please check this for me whether we need to do softmax before reshaping the output from line 101-103? Because softmax is a weighted score of each

X = X.view(batch_size, seq_len, self.nb_tags) # X [batch_size*seq_len, nb_tags] ->[batch_size, seq_len, nb_tags]
X = F.log_softmax(X, dim=1)