Derivative of the covariance matrix with respect to X.

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