error in torch.stack(outputs) about coarse2fine HOT 6 CLOSED

def forward(self, emb, context, state):
        """
        Forward through the decoder.
        Args:
            input (LongTensor): a sequence of input tokens tensors
                                of size (len x batch x nfeats).
            context (FloatTensor): output(tensor sequence) from the encoder
                        RNN of size (src_len x batch x hidden_size).
            state (FloatTensor): hidden state from the encoder RNN for
                                 initializing the decoder.
        Returns:
            outputs (FloatTensor): a Tensor sequence of output from the decoder
                                   of shape (len x batch x hidden_size).
            state (FloatTensor): final hidden state from the decoder.
            attns (dict of (str, FloatTensor)): a dictionary of different
                                type of attention Tensor from the decoder
                                of shape (src_len x batch).
        """
        # Args Check
        assert isinstance(state, RNNDecoderState)
        # END Args Check

        # Run the forward pass of the RNN.
        hidden, outputs, attns = self._run_forward_pass(emb, context, state)

        # Update the state with the result.
        state.update_state(hidden)

        # Concatenates sequence of tensors along a new dimension.
        outputs = torch.stack(outputs)
        for k in attns:
            attns[k] = torch.stack(attns[k])

        return outputs, state, attns