Compute intra-sentence attention given embedded words.

        Parameters
        ----------
        feature_a : NDArray
            Shape (batch_size, length, hidden_size)

        Returns
        -------
        alpha : NDArray
            Shape (batch_size, length, hidden_size)

def hybrid_forward(self, F, feature_a):
        """
        Compute intra-sentence attention given embedded words.

        Parameters
        ----------
        feature_a : NDArray
            Shape (batch_size, length, hidden_size)

        Returns
        -------
        alpha : NDArray
            Shape (batch_size, length, hidden_size)
        """
        tilde_a = self.intra_attn_emb(feature_a)
        e_matrix = F.batch_dot(tilde_a, tilde_a, transpose_b=True)
        alpha = F.batch_dot(e_matrix.softmax(), tilde_a)
        return alpha