[Dy2Stat] Fix bug for loop: a variable is used and created in loop, but used before created (#29769)

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

@@ -167,7 +167,13 @@ class NameVisitor(gast.NodeVisitor):
                 #       var_a = func2(x)
                 #
 
-                if isinstance(var_name_to_ctxs[name][0], gast.Load):
+                is_created = False
+                for ctx in var_name_to_ctxs[name]:
+                    if isinstance(ctx, gast.Store):
+                        is_created = True
+
+                if isinstance(var_name_to_ctxs[name][0],
+                              gast.Load) and is_created:
                     loop_var_names.add(name)
                     create_var_names.add(name)
 

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

@@ -86,11 +86,15 @@ def for_loop_dyfunc(max_len):
 
 def for_loop_dyfunc2(max_len):
     # Test case: a variable is used and created in loop, but used before created
+    x = fluid.layers.fill_constant(shape=[1, 2], dtype="int32", value=1)
+
     for i in range(max_len):
         if i > 1:
             s = a
         a = 1
-    ret = fluid.layers.fill_constant(shape=[1], dtype="int32", value=s)
+        q, _ = x.shape  # test var x.shape only used but not created in loop
+
+    ret = fluid.layers.fill_constant(shape=[1], dtype="int32", value=s + q)
     return ret