return absl::OkStatus();
  }
  ~SqrtGradientFunction() override {}

 private:
  AbstractTensorHandlePtr sqrt_;
};

class MatMulGradientFunction : public GradientFunction {
 public:
  explicit MatMulGradientFunction(vector<AbstractTensorHandle*> f_inputs,
                                  AttrBuilder f_attrs)
      : forward_inputs_(f_inputs), forward_attrs_(f_attrs) {