std::vector<Output>* grad_outputs);

  // Returns a list mapping whether each node in the graph is reachable
  // from outputs_. Keyed by node id.
  std::vector<bool> GetReachableNodes();

  // Creates the gradient subgraph for a while loop (or just stores
  // `summed_grads` if not all incoming gradients are available yet). All exit
  // nodes (which are the first nodes of a loop encountered in the backwards