def update_gradients_diag(self, dL_dKdiag, X):
        self.variance.gradient = np.sum(dL_dKdiag)
        self.lengthscale.gradient = 0.
Given the derivative of the objective with respect to the diagonal of
dataset/ETHPy150Open SheffieldML/GPy/GPy/kern/src/stationary.py/Stationary.update_gradients_diag