std::function<void(std::function<void()>)>* KernelAndDevice::get_runner()
    const {
  if (runner_) {
    return runner_;
  } else {
    static auto* default_runner =
        new std::function<void(std::function<void()>)>(
            [](const std::function<void()>& f) { f(); });
    return default_runner;
  }
}