Fix bidirectional LSTM concatenation (#249)

        if self.bidirectional:

            def combine_bidir(outs):

                return torch.cat([

                    torch.cat([outs[2 * i], outs[2 * i + 1]], dim=0).view(1, bsz, self.output_units)

                    for i in range(self.num_layers)

                ], dim=0)

                return outs.view(self.num_layers, 2, bsz, -1).transpose(1, 2).contiguous().view(self.num_layers, bsz, -1)

            final_hiddens = combine_bidir(final_hiddens)

            final_cells = combine_bidir(final_cells)