model.py

import torch
import torch.nn as nn
from torch.nn.utils.rnn import pack_padded_sequence

class BiLSTMClassifier(nn.Module):
    def __init__(self, vocab_size, embedding_dim, hidden_dim, num_layers=2, dropout=0.3):
        super(BiLSTMClassifier, self).__init__()
        self.embedding = nn.Embedding(vocab_size, embedding_dim, padding_idx=0)
        self.lstm = nn.LSTM(
            embedding_dim,
            hidden_dim,
            num_layers=num_layers,
            batch_first=True,
            bidirectional=True,
            dropout=dropout
        )
        self.dropout = nn.Dropout(dropout)
        self.fc1 = nn.Linear(hidden_dim * 2, hidden_dim)
        self.relu = nn.ReLU()
        self.fc2 = nn.Linear(hidden_dim, 1)

    def forward(self, x, lengths):
        embedded = self.embedding(x)
        packed_embedded = pack_padded_sequence(embedded, lengths.cpu(), batch_first=True, enforce_sorted=False)
        packed_output, (hidden, cell) = self.lstm(packed_embedded)

        # Reshape to separate layers and directions
        hidden = hidden.view(self.lstm.num_layers, 2, -1, self.lstm.hidden_size)
        last_layer_hidden = hidden[-1]

        # Concat Forward + Backward
        cat_hidden = torch.cat((last_layer_hidden[0], last_layer_hidden[1]), dim=1)

        # Classification Head
        x = self.dropout(cat_hidden)
        x = self.fc1(x)
        x = self.relu(x)
        x = self.dropout(x)
        return self.fc2(x)