One step decoding of the translation model.

        Parameters
        ----------
        step_input : NDArray
            Shape (batch_size,)
        states : list of NDArrays

        Returns
        -------
        step_output : NDArray
            Shape (batch_size, C_out)
        states : list
        step_additional_outputs : list
            Additional outputs of the step, e.g, the attention weights

def decode_step(self, step_input, states):
        """One step decoding of the translation model.

        Parameters
        ----------
        step_input : NDArray
            Shape (batch_size,)
        states : list of NDArrays

        Returns
        -------
        step_output : NDArray
            Shape (batch_size, C_out)
        states : list
        step_additional_outputs : list
            Additional outputs of the step, e.g, the attention weights
        """
        step_output, states, step_additional_outputs =\
            self.decoder(self.tgt_embed(step_input), states)
        step_output = self.tgt_proj(step_output)
        return step_output, states, step_additional_outputs