// of `outputs_` that do not get transitively consumed by other `outputs_`.
  // Used to identify nodes at which to stop backprop.
  std::unordered_set<int> GetStopBackpropNodes(
      const std::vector<bool>& reachable_nodes,
      const std::unordered_set<int>& output_nodes) const;

  const Scope& scope_;
  const ops::GradOpRegistry* registry_;