message: Puts the iterations_per_loop, which controls how many steps the TPU system to
  run for each training/evaluatoin loop, into local variable collection.
diff: @@ -92,7 +92,8 @@ def _create_global_step(graph):
  def _create_or_get_iterations_per_loop():
  graph = ops.get_default_graph()
  - iter_vars = graph.get_collection(_TPU_ESTIMATOR)
  + collection_name = '{}_{}'.format(_TPU_ESTIMATOR, _ITERATIONS_PER_LOOP_VAR)
  + iter_vars = graph.get_collection(collection_name)
  if len(iter_vars) == 1:
  return iter_vars[0]
  elif len(iter_vars) > 1:
  @@ -107,7 +108,7 @@ def _create_or_get_iterations_per_loop():
  shape=[],
  dtype=dtypes.int32,
  trainable=False,
  - collections=[_TPU_ESTIMATOR],
  + collections=[collection_name, ops.GraphKeys.LOCAL_VARIABLES],
  use_resource=True)
  