Construct a tensorflow function to compute the bound on the marginal
        likelihood. For a derivation of the terms in here, see the associated
        SGPR notebook.

def _build_likelihood(self):
        """
        Construct a tensorflow function to compute the bound on the marginal
        likelihood. For a derivation of the terms in here, see the associated
        SGPR notebook.
        """

        num_inducing = len(self.feature)
        num_data = tf.cast(tf.shape(self.Y)[0], settings.float_type)
        output_dim = tf.cast(tf.shape(self.Y)[1], settings.float_type)

        err = self.Y - self.mean_function(self.X)
        Kdiag = self.kern.Kdiag(self.X)
        Kuf = features.Kuf(self.feature, self.kern, self.X)
        Kuu = features.Kuu(self.feature, self.kern, jitter=settings.numerics.jitter_level)
        L = tf.cholesky(Kuu)
        sigma = tf.sqrt(self.likelihood.variance)

        # Compute intermediate matrices
        A = tf.matrix_triangular_solve(L, Kuf, lower=True) / sigma
        AAT = tf.matmul(A, A, transpose_b=True)
        B = AAT + tf.eye(num_inducing, dtype=settings.float_type)
        LB = tf.cholesky(B)
        Aerr = tf.matmul(A, err)
        c = tf.matrix_triangular_solve(LB, Aerr, lower=True) / sigma

        # compute log marginal bound
        bound = -0.5 * num_data * output_dim * np.log(2 * np.pi)
        bound += tf.negative(output_dim) * tf.reduce_sum(tf.log(tf.matrix_diag_part(LB)))
        bound -= 0.5 * num_data * output_dim * tf.log(self.likelihood.variance)
        bound += -0.5 * tf.reduce_sum(tf.square(err)) / self.likelihood.variance
        bound += 0.5 * tf.reduce_sum(tf.square(c))
        bound += -0.5 * output_dim * tf.reduce_sum(Kdiag) / self.likelihood.variance
        bound += 0.5 * output_dim * tf.reduce_sum(tf.matrix_diag_part(AAT))

        return bound