!12073 modify asyn ckpt

From: @changzherui
Reviewed-by: @kingxian,@zhoufeng54
Signed-off-by: @kingxian

mindspore/train/callback/_checkpoint.py

@@ -280,7 +280,10 @@ class ModelCheckpoint(Callback):
                 os.remove(graph_file_name)
             _save_graph(cb_params.train_network, graph_file_name)
             self._graph_saved = True
-
+        thread_list = threading.enumerate()
+        for thread in thread_list:
+            if thread.getName() == "asyn_save_ckpt":
+                thread.join()
         self._save_ckpt(cb_params)
 
     def end(self, run_context):
@@ -295,10 +298,9 @@ class ModelCheckpoint(Callback):
         self._save_ckpt(cb_params, _to_save_last_ckpt)
 
         thread_list = threading.enumerate()
-        if len(thread_list) > 1:
-            for thread in thread_list:
-                if thread.getName() == "asyn_save_ckpt":
-                    thread.join()
+        for thread in thread_list:
+            if thread.getName() == "asyn_save_ckpt":
+                thread.join()
 
         from mindspore.parallel._cell_wrapper import destroy_allgather_cell
         destroy_allgather_cell()