[Dy2Stat] Fix undefined var used in For (#32153)

* fix undefind var in For

* fix code style

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

@@ -238,11 +238,16 @@ def is_required_ctx(ctxs, required_ctx):
         return new_name_ids
 
     def _is_call_func_name_node(self, node):
+        white_func_names = set(['append', 'extend'])
         if len(self.ancestor_nodes) > 1:
             assert self.ancestor_nodes[-1] == node
             parent_node = self.ancestor_nodes[-2]
             if isinstance(parent_node, gast.Call) and parent_node.func == node:
-                return True
+                # e.g: var_list.append(elem), var_list is also a name_id.
+                should_skip = isinstance(
+                    node, gast.Attribute) and node.attr in white_func_names
+                if not should_skip:
+                    return True
         return False
 
     def _update_name_ids(self, new_name_ids):
@@ -398,10 +403,13 @@ def _modified_vars(child_dict, parent_dict):
         ])
 
     def _vars_loaded_before_store(ids_dict):
+        """
+        gast.Param is also a kind of `load` semantic.
+        """
         new_dict = defaultdict(list)
         for k, ctxs in six.iteritems(ids_dict):
             for ctx in ctxs:
-                if isinstance(ctx, gast.Load):
+                if isinstance(ctx, (gast.Load, gast.Param)):
                     new_dict[k].append(ctx)
                 elif isinstance(ctx, gast.Store):
                     break

python/paddle/fluid/tests/unittests/dygraph_to_static/test_ifelse.py

@@ -342,5 +342,28 @@ def init_net(self):
         self.Net = DiffModeNet2
 
 
+class TestNewVarCreateInOneBranch(unittest.TestCase):
+    def test_var_used_in_another_for(self):
+        def case_func(training):
+            # targets and targets_list is dynamically defined by training
+            if training:
+                targets = [1, 2, 3]
+                targets_list = [targets]
+
+            num_step = 3
+            for i in range(num_step):
+                if i > 0:
+                    rois, rosi_num = 1, 2
+                    # targets is in loop_vars.
+                    if training:
+                        ros, rosi_num, targets = -1, -2, [-1, -2, -3]
+                        targets_list.append(targets)
+
+            return rosi_num
+
+        self.assertEqual(paddle.jit.to_static(case_func)(False), 2)
+        self.assertEqual(paddle.jit.to_static(case_func)(True), -2)
+
+
 if __name__ == '__main__':
     unittest.main()