message: removing extra copy on unordered_set
diff: @@ -96,7 +96,7 @@ class SymbolicGradientBuilder {
  // Used to identify nodes at which to stop backprop.
  std::unordered_set<int> GetStopBackpropNodes(
  const std::vector<bool>& reachable_nodes,
  - std::unordered_set<int> output_nodes);
  + const std::unordered_set<int>& output_nodes);
  const Scope& scope_;
  const ops::GradOpRegistry* registry_;
  @@ -191,7 +191,7 @@ std::vector<bool> SymbolicGradientBuilder::GetReachableNodes() {
  std::unordered_set<int> SymbolicGradientBuilder::GetStopBackpropNodes(
  const std::vector<bool>& reachable_nodes,
  - std::unordered_set<int> output_nodes) {
  + const std::unordered_set<int>& output_nodes) {
  // Output nodes that get transitively consumed by other `outputs_` are stored
  // in `internal_outputs`.
  std::unordered_set<int> internal_outputs;
  