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,

  *result = outputs[0];
  return absl::OkStatus();
}
}  // namespace

absl::Status GradientRegistry::Register(
    const string& op_name, GradientFunctionFactory gradient_function_factory) {
  auto iter = registry_.find(op_name);
  if (iter != registry_.end()) {
    return absl::AlreadyExistsError(
        absl::StrCat("Gradient already exists for op: ", op_name, "."));
  }