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

}  // namespace

GradientFunction* AddRegisterer(const ForwardOperation& op) {
  return new AddGradientFunction;
}

GradientFunction* ExpRegisterer(const ForwardOperation& op) {
  return new ExpGradientFunction(op.outputs[0]);
}