for (int i = 0, end = target_tensor_ids.size(); i < end; ++i) {
    const int64_t id = target_tensor_ids[i];
    if (output_gradients.empty() || output_gradients[i] == nullptr) {
      auto tensor_it = tensor_tape.find(id);
      if (tensor_it != tensor_tape.end() && tensor_it->second != -1) {
        auto op_it = op_tape.find(tensor_it->second);
        if (op_it == op_tape.end()) {
          return errors::Internal(

                                                          size_t proto_len,
                                                          TF_Status* status);

#ifdef __cplusplus
} /* end extern "C" */
#endif

                                                TF_Status* status);

// ----------------------------------------------------------------

#ifdef __cplusplus
} /* end extern "C" */
#endif