question about decoder inputs from last encoder hidden state in batch translation example

[fancyerii]

In the batch translation example, encoder is:

class EncoderRNN(nn.Module):
    def __init__(self, input_size, hidden_size, n_layers=1, dropout=0.1):
        super(EncoderRNN, self).__init__()
        
        self.input_size = input_size
        self.hidden_size = hidden_size
        self.n_layers = n_layers
        self.dropout = dropout
        
        self.embedding = nn.Embedding(input_size, hidden_size)
        self.gru = nn.GRU(hidden_size, hidden_size, n_layers, dropout=self.dropout, bidirectional=True)
        
    def forward(self, input_seqs, input_lengths, hidden=None):
        embedded = self.embedding(input_seqs)
        packed = torch.nn.utils.rnn.pack_padded_sequence(embedded, input_lengths)
        outputs, hidden = self.gru(packed, hidden)
        outputs, output_lengths = torch.nn.utils.rnn.pad_packed_sequence(outputs)
        outputs = outputs[:, :, :self.hidden_size] + outputs[:, : ,self.hidden_size:]
        return outputs, hidden

encoder = EncoderRNN(input_lang.n_words, hidden_size, n_layers, dropout=dropout)

encoder is a two layer bidirectional GRU.

outputs, hidden = self.gru(packed, hidden)

from pytorch doc: h_n (num_layers * num_directions, batch, hidden_size)

and https://discuss.pytorch.org/t/how-can-i-know-which-part-of-h-n-of-bidirectional-rnn-is-for-backward-process/3883

In my view, hidden=(last_hidden_of_layer0_forward, first_hidden_of_layer0_backward, last_hidden_of_layer1_forward, first_hidden_of_layer1_backward)

But decoder_hidden's initial value seems wrong:

decoder_hidden = encoder_hidden[:decoder.n_layers] # Use last (forward) hidden state from encoder

As the comment we use last(forward) hidden state of encoder. we should use:

decoder_hidden = encoder_hidden[::2] # econder_hidden[0,2] is (last_hidden_of_layer0_forward,last_hidden_of_layer1_forward)

[JingxinLee]

encoder_hidden[:decoder.n_layers] If decoder.n_layer=1, encoder_hidden[:1] is the first hidden state. not the LAST hidden state, right?