!8964 Fix pynative grad core dump bug

From: @zjun3021
Reviewed-by: @kisnwang,@chujinjin
Signed-off-by: @chujinjin

mindspore/ccsrc/pipeline/pynative/pynative_execute.cc

@@ -2022,8 +2022,9 @@ void PynativeExecutor::Clear(const std::string &flag) {
     ConfigManager::GetInstance().ResetIterNum();
     if (top_graph_cells_.find(flag) != top_graph_cells_.end()) {
       op_forward_map_.clear();
+      Clean();
     }
-    Clean();
+    node_abs_map_.clear();
     return;
   }