code: def dKdiag_dX(self, dL_dKdiag, X, target):
          
          self._K_diag_computations(X)
          arg = self._K_diag_poly_arg
          target += 2.*self.weight_variance*self.degree*self.variance*X*dL_dKdiag[:, None]*(arg**(self.degree-1))[:, None]
docs: Gradient of diagonal of covariance with respect to X