Listen-Attend-Spell | PyTorch implementation of Listen , Attend and Spell | Speech library

Setting bidirectional=True makes the LSTM bidirectional, which means there will be two LSTMs, one that goes from left to right and the other that goes from right to left.

From the nn.LSTM documentation - Outputs:

• output of shape (seq_len, batch, num_directions * hidden_size): tensor containing the output features (h_t) from the last layer of the LSTM, for each t. If a torch.nn.utils.rnn.PackedSequence has been given as the input, the output will also be a packed sequence.

  For the unpacked case, the directions can be separated using output.view(seq_len, batch, num_directions, hidden_size), with forward and backward being direction 0 and 1 respectively. Similarly, the directions can be separated in the packed case.

Your output has the size [batch, seq_len, 2 * hidden_size] (batch and seq_len are swapped in your case due to setting batch_first=True) because of using a bidirectional LSTM. The outputs of the two are concatenated in order to have the information of both, which you could easily separate if you wanted to treat them differently.