// clear the op_tape. We can clear the tape because ownership of
    // backward functions that will be used for gradient computation
    // has been transferred to `result`.
    for (const auto& op_pair : *op_tape) {
      op_pair.second.backward_function_deleter(
          op_pair.second.backward_function);
    }
    op_tape->clear();
  }
  return result;
}