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

    :return: NxQ1xQ2

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(.), m2(.) :: Linear mean functions

    :return: NxQ1xQ2
    """
    with params_as_tensors_for(mean1, mean2):
        e_xxt = p.cov + (p.mu[:, :, None] * p.mu[:, None, :])  # NxDxD
        e_A1t_xxt_A2 = tf.einsum("iq,nij,jz->nqz", mean1.A, e_xxt, mean2.A)  # NxQ1xQ2
        e_A1t_x_b2t = tf.einsum("iq,ni,z->nqz", mean1.A, p.mu, mean2.b)  # NxQ1xQ2
        e_b1_xt_A2 = tf.einsum("q,ni,iz->nqz", mean1.b, p.mu, mean2.A)  # NxQ1xQ2
        e_b1_b2t = mean1.b[:, None] * mean2.b[None, :]  # Q1xQ2

        return e_A1t_xxt_A2 + e_A1t_x_b2t + e_b1_xt_A2 + e_b1_b2t