message: Fix a TransferFromDeviceToDevice race in the client.
diff: @@ -380,14 +380,17 @@ StatusOr<std::unique_ptr<PyTpuBuffer>> PyTpuBuffer::CopyToDevice(
  }
  tpu_driver::TpuDriver* driver = client_->driver();
  - tpu_driver::BufferHandle* src_handle = src_device_buffer->handle.get();
  TF_ASSIGN_OR_RETURN(
  std::unique_ptr<PyTpuBuffer> dst_buffer,
  CreateBuffer(
  on_host_shape_,
  - [driver, src_handle](tpu_driver::BufferHandle* dst_handle) {
  - return driver->TransferFromDeviceToDevice(src_handle, dst_handle,
  - {});
  + [driver, src_device_buffer](tpu_driver::BufferHandle* dst_handle) {
  + std::vector<tpu_driver::Event*> src_wait_for_use;
  + for (auto& event : src_device_buffer->wait_for_use) {
  + src_wait_for_use.push_back(event.get());
  + }
  + return driver->TransferFromDeviceToDevice(
  + src_device_buffer->handle.get(), dst_handle, src_wait_for_use);
  },
  client_, dst_device_ordinal));
  // TODO(jiawenhao): This may be too pessimistic: it prevents future readers
  