code: def update_gradients_direct(self, dL_dVar, dL_dLen):
          
          self.variance.gradient = dL_dVar
          self.lengthscale.gradient = dL_dLen
docs: 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.