def update_gradients_diag(self, dL_dKdiag, X):
self.variance.gradient = np.sum(dL_dKdiag)<EOL>self.lengthscale.gradient = <NUM_LIT:0.><EOL>
Given the derivative of the objective with respect to the diagonal of
the covariance matrix, compute the derivative wrt the parameters of
this kernel and stor in the <parameter>.gradient field.

See also update_gradients_full