Parameters
        ----------
        inputs : ``PackedSequence``, required.
            A batch first ``PackedSequence`` to run the stacked LSTM over.
        initial_state : Tuple[torch.Tensor, torch.Tensor], optional, (default = None)
            A tuple (state, memory) representing the initial hidden state and memory
            of the LSTM. Each tensor has shape (num_layers, batch_size, output_dimension * 2).

        Returns
        -------
        output_sequence : PackedSequence
            The encoded sequence of shape (batch_size, sequence_length, hidden_size * 2)
        final_states: torch.Tensor
            The per-layer final (state, memory) states of the LSTM, each with shape
            (num_layers * 2, batch_size, hidden_size * 2).

def forward(self,  # pylint: disable=arguments-differ
                inputs: PackedSequence,
                initial_state: Optional[Tuple[torch.Tensor, torch.Tensor]] = None
               ) -> Tuple[PackedSequence, Tuple[torch.Tensor, torch.Tensor]]:
        """
        Parameters
        ----------
        inputs : ``PackedSequence``, required.
            A batch first ``PackedSequence`` to run the stacked LSTM over.
        initial_state : Tuple[torch.Tensor, torch.Tensor], optional, (default = None)
            A tuple (state, memory) representing the initial hidden state and memory
            of the LSTM. Each tensor has shape (num_layers, batch_size, output_dimension * 2).

        Returns
        -------
        output_sequence : PackedSequence
            The encoded sequence of shape (batch_size, sequence_length, hidden_size * 2)
        final_states: torch.Tensor
            The per-layer final (state, memory) states of the LSTM, each with shape
            (num_layers * 2, batch_size, hidden_size * 2).
        """
        if not initial_state:
            hidden_states = [None] * len(self.lstm_layers)
        elif initial_state[0].size()[0] != len(self.lstm_layers):
            raise ConfigurationError("Initial states were passed to forward() but the number of "
                                     "initial states does not match the number of layers.")
        else:
            hidden_states = list(zip(initial_state[0].split(1, 0),
                                     initial_state[1].split(1, 0)))

        output_sequence = inputs
        final_h = []
        final_c = []
        for i, state in enumerate(hidden_states):
            forward_layer = getattr(self, 'forward_layer_{}'.format(i))
            backward_layer = getattr(self, 'backward_layer_{}'.format(i))
            # The state is duplicated to mirror the Pytorch API for LSTMs.
            forward_output, final_forward_state = forward_layer(output_sequence, state)
            backward_output, final_backward_state = backward_layer(output_sequence, state)

            forward_output, lengths = pad_packed_sequence(forward_output, batch_first=True)
            backward_output, _ = pad_packed_sequence(backward_output, batch_first=True)

            output_sequence = torch.cat([forward_output, backward_output], -1)
            # Apply layer wise dropout on each output sequence apart from the
            # first (input) and last
            if i < (self.num_layers - 1):
                output_sequence = self.layer_dropout(output_sequence)
            output_sequence = pack_padded_sequence(output_sequence, lengths, batch_first=True)

            final_h.extend([final_forward_state[0], final_backward_state[0]])
            final_c.extend([final_forward_state[1], final_backward_state[1]])

        final_h = torch.cat(final_h, dim=0)
        final_c = torch.cat(final_c, dim=0)
        final_state_tuple = (final_h, final_c)
        return output_sequence, final_state_tuple