Compute the expectation:
    expectation[n] = <m1(x_n)^T m2(x_n)>_p(x_n)
        - m1(.) :: Linear mean function
        - m2(.) :: Identity mean function

    :return: NxQxD

def _expectation(p, mean1, none1, mean2, none2, nghp=None):
    """
    Compute the expectation:
    expectation[n] = <m1(x_n)^T m2(x_n)>_p(x_n)
        - m1(.) :: Linear mean function
        - m2(.) :: Identity mean function

    :return: NxQxD
    """
    with params_as_tensors_for(mean1):
        N = tf.shape(p.mu)[0]
        e_xxt = p.cov + (p.mu[:, :, None] * p.mu[:, None, :])  # NxDxD
        e_A_xxt = tf.matmul(tf.tile(mean1.A[None, ...], (N, 1, 1)), e_xxt, transpose_a=True)  # NxQxD
        e_b_xt = mean1.b[None, :, None] * p.mu[:, None, :]  # NxQxD

        return e_A_xxt + e_b_xt