message: Fix the bug that sometime cluster_spec becomes None in parameter server strategy.
diff: @@ -40,12 +40,14 @@ _LOCAL_GPU_0 = "/device:GPU:0"
  def _normalize_cluster_spec(cluster_spec):
  + """Makes `cluster_spec` into a `ClusterSpec` object."""
  if isinstance(cluster_spec, (dict, cluster_pb2.ClusterDef)):
  return server_lib.ClusterSpec(cluster_spec)
  elif not isinstance(cluster_spec, server_lib.ClusterSpec):
  raise ValueError(
  "`cluster_spec' should be dict or a `tf.train.ClusterSpec` or a "
  "`tf.train.ClusterDef` object")
  + return cluster_spec
  # TODO(yuefengz): maybe cache variables on local CPU.
  