[Dy2Stat] Fix ForLoop Transformation with single return (#40683)

* [Dy2Stat] Fix ForLoop Transformation with single return

* [Dy2Stat] Fix ForLoop Transformation with single return

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

@@ -693,7 +693,7 @@ class LoopTransformer(gast.NodeTransformer):
         new_body = node.body
         new_body.append(
             gast.Return(value=generate_name_node(
-                loop_var_names, ctx=gast.Load())))
+                loop_var_names, ctx=gast.Load(), gen_tuple_if_single=True)))
         body_func_node = gast.FunctionDef(
             name=unique_name.generate(WHILE_BODY_PREFIX),
             args=gast.arguments(

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

@@ -121,6 +121,13 @@ def for_loop_dyfunc_not_support(max_len):
     return ret
 
 
+def for_break_single_return(max_len):
+    for i in range(3):
+        if i == 2:
+            break
+    return i
+
+
 def while_loop_bool_op(x):
     i = fluid.dygraph.to_variable(x)
 
@@ -297,7 +304,10 @@ class TestTransformWhileLoop(unittest.TestCase):
                 ret = declarative(self.dyfunc)(tensor_x)
             else:
                 ret = self.dyfunc(tensor_x)
-            return ret.numpy()
+            if hasattr(ret, "numpy"):
+                return ret.numpy()
+            else:
+                return ret
 
     def test_ast_to_func(self):
         static_numpy = self._run_static()
@@ -320,6 +330,11 @@ class TestTransformWhileLoopWithNone(TestTransformWhileLoop):
         self.dyfunc = while_loop_dyfunc_with_none
 
 
+class TestForBreakSingleReturn(TestTransformWhileLoop):
+    def _init_dyfunc(self):
+        self.dyfunc = for_break_single_return
+
+
 class TestWhileLoopBoolOp(TestTransformWhileLoop):
     def _init_dyfunc(self):
         self.dyfunc = while_loop_bool_op