message: In TPUStrategy.configure, copy cluster spec from cluster resolver so that the user doesn't have to pass it again to session_config.
diff: @@ -320,3 +320,7 @@ class TPUStrategy(one_device_strategy.OneDeviceStrategy):
  del cluster_spec, task_type, task_id
  if session_config:
  session_config.isolate_session_state = True
  + cluster_spec = self._tpu_cluster_resolver.cluster_spec()
  + if cluster_spec:
  + session_config.cluster_def.CopyFrom(cluster_spec.as_cluster_def())
  +
  