code: def dpsi2_dmuS(self, dL_dpsi2, Z, mu, S, target_mu, target_S):
          
          self._psi_computations(Z, mu, S)
          tmp = (self.inv_lengthscale2 * self._psi2[:, :, :, None]) / self._psi2_denom
          target_mu += -2.*(dL_dpsi2[:, :, :, None] * tmp * self._psi2_mudist).sum(1).sum(1)
          target_S += (dL_dpsi2[:, :, :, None] * tmp * (2.*self._psi2_mudist_sq - 1)).sum(1).sum(1)
docs: Think N,num_inducing,num_inducing,input_dim