!10703 fix the error of removing meta file in pynative mode

From: @caozhou_huawei
Reviewed-by: @changzherui,@zhunaipan,@zh_qh
Signed-off-by: @zh_qh

mindspore/train/callback/_checkpoint.py

@@ -275,7 +275,7 @@ class ModelCheckpoint(Callback):
         # save graph (only once)
         if not self._graph_saved:
             graph_file_name = os.path.join(self._directory, self._prefix + '-graph.meta')
-            if os.path.isfile(graph_file_name):
+            if os.path.isfile(graph_file_name) and context.get_context("mode") == context.GRAPH_MODE:
                 os.remove(graph_file_name)
             _save_graph(cb_params.train_network, graph_file_name)
             self._graph_saved = True