fix bug of ListTransformer in dygraph_to_static. test=develop (#23153)

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

@@ -196,6 +196,7 @@ class ListTransformer(gast.NodeTransformer):
             self.list_name_to_updated[target_id] = False
             self.list_nodes.add(node)
             return True
-        elif target_id in self.list_name_to_updated:
+        elif target_id in self.list_name_to_updated and \
+                self.list_name_to_updated[target_id] == False:
             del self.list_name_to_updated[target_id]
         return False

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

@@ -58,8 +58,8 @@ def test_list_in_for_loop_with_concat(x, iter_num):
     a = []
     for i in range(iter_num):
         a.append(x)
-    out = fluid.layers.concat(a, axis=0)
-    return out
+    a = fluid.layers.concat(a, axis=0)
+    return a
 
 
 def test_list_in_while_loop(x, iter_num):