message: Adding manual initialization of _device_fn in legacy RunConfig class
diff: @@ -299,6 +299,7 @@ class RunConfig(ClusterConfig, core_run_config.RunConfig):
  # so instead of breaking compatibility with that assumption, we
  # just manually initialize this field:
  self._distribute = None
  + self._device_fn = None
  gpu_options = config_pb2.GPUOptions(
  per_process_gpu_memory_fraction=gpu_memory_fraction)
  