def update_gradients_diag(self, dL_dKdiag, X):
        """derivative of the diagonal of the covariance matrix with respect to the parameters."""
        self.variance.gradient = np.sum(dL_dKdiag)
        self.period.gradient = 0
        self.lengthscale.gradient = 0
derivative of the diagonal of the covariance matrix with respect to the parameters.