message: Remove the implementation of TPU Strategy initialize and finalize
diff: @@ -51,9 +51,6 @@ from tensorflow.python.platform import tf_logging as logging
  from tensorflow.python.util import nest
  -_TPU_INITIALIZE_SYSTEM_COLLECTION = "TPU_STRATEGY_INITIALIZE"
  -
  -
  def initialize_tpu_system(cluster_resolver=None):
  """Initialize the TPU devices in a separate session and graph.
  @@ -463,28 +460,6 @@ class TPUExtended(distribute_lib.DistributionStrategyExtended):
  """
  initialize_tpu_system(self._tpu_cluster_resolver)
  - def _initialize(self):
  - if context.executing_eagerly():
  - # TODO(priyag): Add appopriate call here when eager is supported for TPUs.
  - raise NotImplementedError("Eager mode not supported in TPUStrategy.")
  - else:
  - # TODO(jhseu): We need this hack because DistributionStrategies must be
  - # pickleable for copy.deepcopy(). Remove when initialize_system goes away.
  - graph = ops.get_default_graph()
  - tpu_init = graph.get_collection(_TPU_INITIALIZE_SYSTEM_COLLECTION)
  - if tpu_init:
  - return tpu_init
  - graph.add_to_collection(_TPU_INITIALIZE_SYSTEM_COLLECTION,
  - tpu.initialize_system())
  - return graph.get_collection(_TPU_INITIALIZE_SYSTEM_COLLECTION)
  -
  - def _finalize(self):
  - if context.executing_eagerly():
  - # TODO(priyag): Add appopriate call here when eager is supported for TPUs.
  - raise NotImplementedError("Eager mode not supported in TPUStrategy.")
  - else:
  - return []
  -
  def _create_variable(self, next_creator, *args, **kwargs):
  """Create a TPUMirroredVariable. See `DistributionStrategy.scope`."""
  colocate_with = kwargs.pop("colocate_with", None)
  