message: Support packed TensorHandles on CustomDevices.
diff: @@ -316,8 +316,7 @@ Status TensorHandle::CreatePackedHandle(std::vector<TensorHandle*>&& handles,
  std::vector<string> devices;
  for (auto* handle : handles) {
  if (VariantDeviceIsCustom(handle->device())) {
  - return errors::InvalidArgument(
  - "CustomDevice is not supported for packing.");
  + devices.push_back(absl::get<CustomDevice*>(handle->device())->name());
  } else {
  devices.push_back(handle->op_device() ? handle->op_device()->name()
  : ctx->HostCPU()->name());
  