seq2seq.py

import torch
import torch.nn as nn
import torch.optim as optim
from torchtext.datasets import Multi30k
from torchtext.data import Field, BucketIterator
import numpy as np
import spacy
import random
from torch.utils.tensorboard import SummaryWriter  # to print to tensorboard
from utils import translate_sentence, bleu, save_checkpoint, load_checkpoint

spacy_ger = spacy.load("de")
spacy_eng = spacy.load("en")


def tokenize_ger(text):
    return [tok.text for tok in spacy_ger.tokenizer(text)]


def tokenize_eng(text):
    return [tok.text for tok in spacy_eng.tokenizer(text)]


german = Field(tokenize=tokenize_ger, lower=True, init_token="<sos>", eos_token="<eos>")

english = Field(
    tokenize=tokenize_eng, lower=True, init_token="<sos>", eos_token="<eos>"
)

train_data, valid_data, test_data = Multi30k.splits(
    exts=(".de", ".en"), fields=(german, english)
)

german.build_vocab(train_data, max_size=10000, min_freq=2)
english.build_vocab(train_data, max_size=10000, min_freq=2)


class Encoder(nn.Module):
    def __init__(self, input_size, embedding_size, hidden_size, num_layers, p):
        super(Encoder, self).__init__()
        self.dropout = nn.Dropout(p)
        self.hidden_size = hidden_size
        self.num_layers = num_layers

        self.embedding = nn.Embedding(input_size, embedding_size)
        self.rnn = nn.LSTM(embedding_size, hidden_size, num_layers, dropout=p)

    def forward(self, x):
        # x shape: (seq_length, N) where N is batch size

        embedding = self.dropout(self.embedding(x))
        # embedding shape: (seq_length, N, embedding_size)

        outputs, (hidden, cell) = self.rnn(embedding)
        # outputs shape: (seq_length, N, hidden_size)

        return hidden, cell


class Decoder(nn.Module):
    def __init__(
        self, input_size, embedding_size, hidden_size, output_size, num_layers, p
    ):
        super(Decoder, self).__init__()
        self.dropout = nn.Dropout(p)
        self.hidden_size = hidden_size
        self.num_layers = num_layers

        self.embedding = nn.Embedding(input_size, embedding_size)
        self.rnn = nn.LSTM(embedding_size, hidden_size, num_layers, dropout=p)
        self.fc = nn.Linear(hidden_size, output_size)

    def forward(self, x, hidden, cell):
        # x shape: (N) where N is for batch size, we want it to be (1, N), seq_length
        # is 1 here because we are sending in a single word and not a sentence
        x = x.unsqueeze(0)

        embedding = self.dropout(self.embedding(x))
        # embedding shape: (1, N, embedding_size)

        outputs, (hidden, cell) = self.rnn(embedding, (hidden, cell))
        # outputs shape: (1, N, hidden_size)

        predictions = self.fc(outputs)

        # predictions shape: (1, N, length_target_vocabulary) to send it to
        # loss function we want it to be (N, length_target_vocabulary) so we're
        # just gonna remove the first dim
        predictions = predictions.squeeze(0)

        return predictions, hidden, cell


class Seq2Seq(nn.Module):
    def __init__(self, encoder, decoder):
        super(Seq2Seq, self).__init__()
        self.encoder = encoder
        self.decoder = decoder

    def forward(self, source, target, teacher_force_ratio=0.5):
        batch_size = source.shape[1]
        target_len = target.shape[0]
        target_vocab_size = len(english.vocab)

        outputs = torch.zeros(target_len, batch_size, target_vocab_size).to(device)

        hidden, cell = self.encoder(source)

        # Grab the first input to the Decoder which will be <SOS> token
        x = target[0]

        for t in range(1, target_len):
            # Use previous hidden, cell as context from encoder at start
            output, hidden, cell = self.decoder(x, hidden, cell)

            # Store next output prediction
            outputs[t] = output

            # Get the best word the Decoder predicted (index in the vocabulary)
            best_guess = output.argmax(1)

            # With probability of teacher_force_ratio we take the actual next word
            # otherwise we take the word that the Decoder predicted it to be.
            # Teacher Forcing is used so that the model gets used to seeing
            # similar inputs at training and testing time, if teacher forcing is 1
            # then inputs at test time might be completely different than what the
            # network is used to. This was a long comment.
            x = target[t] if random.random() < teacher_force_ratio else best_guess

        return outputs


### We're ready to define everything we need for training our Seq2Seq model ###

# Training hyperparameters
num_epochs = 100
learning_rate = 0.001
batch_size = 64

# Model hyperparameters
load_model = False
device = torch.device("cuda" if torch.cuda.is_available() else "cpu")
input_size_encoder = len(german.vocab)
input_size_decoder = len(english.vocab)
output_size = len(english.vocab)
encoder_embedding_size = 300
decoder_embedding_size = 300
hidden_size = 1024  # Needs to be the same for both RNN's
num_layers = 2
enc_dropout = 0.5
dec_dropout = 0.5

# Tensorboard to get nice loss plot
writer = SummaryWriter(f"runs/loss_plot")
step = 0

train_iterator, valid_iterator, test_iterator = BucketIterator.splits(
    (train_data, valid_data, test_data),
    batch_size=batch_size,
    sort_within_batch=True,
    sort_key=lambda x: len(x.src),
    device=device,
)

encoder_net = Encoder(
    input_size_encoder, encoder_embedding_size, hidden_size, num_layers, enc_dropout
).to(device)

decoder_net = Decoder(
    input_size_decoder,
    decoder_embedding_size,
    hidden_size,
    output_size,
    num_layers,
    dec_dropout,
).to(device)

model = Seq2Seq(encoder_net, decoder_net).to(device)
optimizer = optim.Adam(model.parameters(), lr=learning_rate)

pad_idx = english.vocab.stoi["<pad>"]
criterion = nn.CrossEntropyLoss(ignore_index=pad_idx)

if load_model:
    load_checkpoint(torch.load("my_checkpoint.pth.tar"), model, optimizer)


sentence = "ein boot mit mehreren männern darauf wird von einem großen pferdegespann ans ufer gezogen."

for epoch in range(num_epochs):
    print(f"[Epoch {epoch} / {num_epochs}]")

    checkpoint = {"state_dict": model.state_dict(), "optimizer": optimizer.state_dict()}
    save_checkpoint(checkpoint)

    model.eval()

    translated_sentence = translate_sentence(
        model, sentence, german, english, device, max_length=50
    )

    print(f"Translated example sentence: \n {translated_sentence}")

    model.train()

    for batch_idx, batch in enumerate(train_iterator):
        # Get input and targets and get to cuda
        inp_data = batch.src.to(device)
        target = batch.trg.to(device)

        # Forward prop
        output = model(inp_data, target)

        # Output is of shape (trg_len, batch_size, output_dim) but Cross Entropy Loss
        # doesn't take input in that form. For example if we have MNIST we want to have
        # output to be: (N, 10) and targets just (N). Here we can view it in a similar
        # way that we have output_words * batch_size that we want to send in into
        # our cost function, so we need to do some reshapin. While we're at it
        # Let's also remove the start token while we're at it
        output = output[1:].reshape(-1, output.shape[2])
        target = target[1:].reshape(-1)

        optimizer.zero_grad()
        loss = criterion(output, target)

        # Back prop
        loss.backward()

        # Clip to avoid exploding gradient issues, makes sure grads are
        # within a healthy range
        torch.nn.utils.clip_grad_norm_(model.parameters(), max_norm=1)

        # Gradient descent step
        optimizer.step()

        # Plot to tensorboard
        writer.add_scalar("Training loss", loss, global_step=step)
        step += 1


score = bleu(test_data[1:100], model, german, english, device)
print(f"Bleu score {score*100:.2f}")