// output of SumGradients(), not the input (i.e. each exit node may have
  // multiple incoming gradients, but we only store the combined Output here).
  std::map<WhileContext*, std::map<Node*, Output>> while_backprops_;

  SymbolicGradientBuilder(const SymbolicGradientBuilder&) = delete;
  void operator=(const SymbolicGradientBuilder&) = delete;
};

SymbolicGradientBuilder::SymbolicGradientBuilder(