default_cancellation_manager_(absl::make_unique<CancellationManager>()) {
  device_threads_.reserve(devices.size());
  for (int device_index = 0; device_index < devices.size(); ++device_index) {
    device_threads_.emplace_back(new DeviceThread(
        devices[device_index].c_str(), is_async, in_flight_nodes_limit));
  }
}

// Necessary for a unique_ptr to a forward-declared type.

        treated as tensors, so if you still want them to be treated that way,
        you need to wrap them with `tf.convert_to_tensor`.
    *   No lowering on gradient case op when input is `DeviceIndex` op.
    *   Extend the ragged version of `tf.gather` to support `batch_dims` and
        `axis` args.
    *   Update `tf.map_fn` to support RaggedTensors and SparseTensors.