fix dygraph2static bugs (#43688)

因此将N次前向共用一个 vec scope 换为了N次前向分别使用自己的一个vecscope

python/paddle/fluid/dygraph/dygraph_to_static/partial_program.py

@@ -152,7 +152,6 @@ class PartialProgramLayer:
         assert isinstance(self._build_strategy, BuildStrategy)
 
         self._origin_main_program = self._verify_program(main_program)
-        self._tmp_scope_vec = self._create_scope_vec()
         self._cuda_graph_vec = self._create_cuda_graph_vec()
         self._cuda_graph_capture_mode = ""
         self._cuda_graph_pool_id = 0
@@ -363,9 +362,8 @@ class PartialProgramLayer:
 
         _C_ops.run_program(self._valid_vars(in_vars),
                            self._valid_vars(self._params),
-                           self._valid_vars(out_vars), self._tmp_scope_vec,
+                           self._valid_vars(out_vars), self._create_scope_vec(),
                            self._double_grads, self._cuda_graph_vec, *attrs)
-        self.drop_scope_if_no_grad()
         restored_nest_out = self._restore_out(out_vars)
         return self._remove_no_value(restored_nest_out)
 
@@ -379,13 +377,6 @@ class PartialProgramLayer:
                     in_vars[i] = var.astype('float16')
                     in_vars[i].name = name
 
-    def drop_scope_if_no_grad(self):
-        tracer = framework._dygraph_tracer()
-        scope = self._tmp_scope_vec.value().get_scope() if isinstance(
-            self._tmp_scope_vec, (core.VarBase)) else self._tmp_scope_vec[0]
-        if self.training and not tracer._has_grad:
-            scope.drop_kids()
-
     @property
     def program(self):
         if self.training: