message: [XLA:Python] Remove host synchronization after host-to-device transfers.
  This synchronization is unnecessary on all of our supported platforms.
diff: @@ -299,9 +299,6 @@ StatusOr<PyLocalBuffer> PyLocalBuffer::FromPython(
  std::move(client), device));
  device.ThenRelease(device.host_to_device_stream(), std::move(py_buffer_ref));
  - if (!device.asynchronous()) {
  - TF_RETURN_IF_ERROR(device.host_to_device_stream()->BlockHostUntilDone());
  - }
  return buffer;
  }
  @@ -360,9 +357,6 @@ PyLocalBuffer::FromPythonValues(
  const Device& device = client->device(device_ordinal);
  device.ThenRelease(device.host_to_device_stream(),
  std::move(transfers[i].py_buffer_refs));
  - if (!device.asynchronous()) {
  - TF_RETURN_IF_ERROR(device.host_to_device_stream()->BlockHostUntilDone());
  - }
  }
  for (int i = 0; i < num_arguments; ++i) {
  @@ -420,10 +414,6 @@ PyLocalBuffer::FromPythonValues(
  device.ThenReleaseOnWorkerThread(device.host_to_device_stream(),
  std::move(tuple_buffer));
  }
  - if (!device.asynchronous()) {
  - TF_RETURN_IF_ERROR(device.host_to_device_stream()->BlockHostUntilDone());
  - }
  -
  return buffer;
  }
  