Execute the encoder.

        :param inputs: tensor with indices from the vocabulary
        :param lengths: vector with sequence lengths (excluding padding)

        returns: tensor with encoded sequences

def forward(self, inputs, lengths):
        """
        Execute the encoder.

        :param inputs: tensor with indices from the vocabulary
        :param lengths: vector with sequence lengths (excluding padding)

        returns: tensor with encoded sequences
        """
        x = self.embedder(inputs)

        # bidirectional layer
        x = self.dropout(x)
        x = pack_padded_sequence(x, lengths.cpu().numpy(),
                                 batch_first=self.batch_first)
        x, _ = self.rnn_layers[0](x)
        x, _ = pad_packed_sequence(x, batch_first=self.batch_first)

        # 1st unidirectional layer
        x = self.dropout(x)
        x, _ = self.rnn_layers[1](x)

        # the rest of unidirectional layers,
        # with residual connections starting from 3rd layer
        for i in range(2, len(self.rnn_layers)):
            residual = x
            x = self.dropout(x)
            x, _ = self.rnn_layers[i](x)
            x = x + residual

        return x