Derivative of the covariance with respect to the parameters.

def _param_grad_helper(self, dL_dK, X, X2, target):
        """Derivative of the covariance with respect to the parameters."""
        if (X2 is None) or (X2 is X):
            dL_dKdiag = dL_dK.flat[::dL_dK.shape[0]+1]
            self.dKdiag_dtheta(dL_dKdiag, X, target)