std::vector<int64_t> skip_input_indices;
  AttrBuilder attrs;
};

using GradientFunctionFactory =
    std::function<GradientFunction*(const ForwardOperation& op)>;

// Map from op name to a `GradientFunctionFactory`.
class GradientRegistry {
 public:
  absl::Status Register(const string& op,
                        GradientFunctionFactory gradient_function_factory);
  absl::Status Lookup(
      const ForwardOperation& op,