Another missed _ref in sync_replicas_optimizer.

Change: 139491940

tensorflow/python/training/sync_replicas_optimizer.py

@@ -955,7 +955,7 @@ class SyncReplicasOptimizer(optimizer.Optimizer):
     if num_tokens > 0:
       with ops.device(self._global_step.device), ops.name_scope(""):
         tokens = array_ops.fill([num_tokens],
-                                self._global_step.ref())
+                                self._global_step._ref())  # pylint: disable=protected-access
         init_tokens = self._sync_token_queue.enqueue_many((tokens,))
     else:
       init_tokens = control_flow_ops.no_op(name="no_init_tokens")