def update_gradients_direct(self, dL_dVar, dL_dLen):
self.variance.gradient = dL_dVar<EOL>self.lengthscale.gradient = dL_dLen<EOL>
Specially intended for the Grid regression case.
Given the computed log likelihood derivates, update the corresponding
kernel and likelihood gradients.
Useful for when gradients have been computed a priori.