for (auto input : forward_inputs_) {
      if (input) {
        input->Unref();
      }
    }
  }

 private:
  // TODO(b/174778737): Only hold needed inputs.
  vector<AbstractTensorHandle*> forward_inputs_;
};

class Log1pGradientFunction : public GradientFunction {
 public:
  explicit Log1pGradientFunction(vector<AbstractTensorHandle*> f_inputs)
      : forward_inputs_(f_inputs) {