This function computes the optimal density for v, q*(v), up to a constant

def _build_likelihood(self):
        """
        This function computes the optimal density for v, q*(v), up to a constant
        """
        # get the (marginals of) q(f): exactly predicting!
        fmean, fvar = self._build_predict(self.X, full_cov=False)
        return tf.reduce_sum(self.likelihood.variational_expectations(fmean, fvar, self.Y))