def decode_seq(self, inputs, states, valid_length=None):
        """Decode the decoder inputs. This function is only used for training.

        Parameters
        ----------
        inputs : NDArray, Shape (batch_size, length, C_in)
        states : list of NDArrays or None
            Initial states. The list of decoder states
        valid_length : NDArray or None
            Valid lengths of each sequence. This is usually used when part of sequence has
            been padded. Shape (batch_size,)

        Returns
        -------
        output : NDArray, Shape (batch_size, length, C_out)
        states : list
            The decoder states, includes:

            - mem_value : NDArray
            - mem_masks : NDArray, optional
        additional_outputs : list of list
            Either be an empty list or contains the attention weights in this step.
            The attention weights will have shape (batch_size, length, mem_length) or
            (batch_size, num_heads, length, mem_length)
        """
        batch_size = inputs.shape[0]
        length = inputs.shape[1]
        length_array = mx.nd.arange(length, ctx=inputs.context, dtype=inputs.dtype)
        mask = mx.nd.broadcast_lesser_equal(
            length_array.reshape((1, -1)),
            length_array.reshape((-1, 1)))
        if valid_length is not None:
            arange = mx.nd.arange(length, ctx=valid_length.context, dtype=valid_length.dtype)
            batch_mask = mx.nd.broadcast_lesser(
                arange.reshape((1, -1)),
                valid_length.reshape((-1, 1)))
            mask = mx.nd.broadcast_mul(mx.nd.expand_dims(batch_mask, -1),
                                       mx.nd.expand_dims(mask, 0))
        else:
            mask = mx.nd.broadcast_axes(mx.nd.expand_dims(mask, axis=0), axis=0, size=batch_size)
        states = [None] + states
        output, states, additional_outputs = self.forward(inputs, states, mask)
        states = states[1:]
        if valid_length is not None:
            output = mx.nd.SequenceMask(output,
                                        sequence_length=valid_length,
                                        use_sequence_length=True,
                                        axis=1)
        return output, states, additional_outputs