message: Fix system metadata retrieval for non-resolvable addresses within TPU Distribution Strategy
diff: @@ -37,7 +37,6 @@ from tensorflow.python.ops import math_ops
  from tensorflow.python.ops import variable_scope as vs
  from tensorflow.python.ops import variables as variables_lib
  from tensorflow.python.training import device_util
  -from tensorflow.python.training import server_lib
  from tensorflow.python.util import nest
  @@ -46,8 +45,8 @@ def get_tpu_system_metadata(tpu_cluster_resolver):
  master = tpu_cluster_resolver.master()
  # pylint: disable=protected-access
  - cluster_def = (tpu_cluster_resolver.cluster_spec()
  - or server_lib.ClusterSpec({})).as_cluster_def()
  + cluster_spec = tpu_cluster_resolver.cluster_spec()
  + cluster_def = cluster_spec.as_cluster_def() if cluster_spec else None
  tpu_system_metadata = (
  tpu_system_metadata_lib._query_tpu_system_metadata(
  master,
  