code: def update_gradients_full(self, dL_dK, X, X2=None):
          
          dvar, dw, db = self._comp_grads(dL_dK, X, X2)[:3]
          self.variance.gradient = dvar
          self.weight_variance.gradient = dw
          self.bias_variance.gradient = db
docs: Derivative of the covariance with respect to the parameters.