[Dy2stat] Fix the bug that loop_body_func may return single element (#31806)

Our old `loop_body` function may return single element when `loop_vars` just contains only 1 element, which can cause bug. The key point of this PR is forcing `loop_body` functions always return tuple.

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

@@ -594,7 +594,7 @@ class LoopTransformer(gast.NodeTransformer):
         # append return values for loop body
         body_stmts.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(FOR_BODY_PREFIX),
             args=gast.arguments(

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

@@ -381,9 +381,15 @@ def get_attribute_full_name(node):
     return astor.to_source(gast.gast_to_ast(node)).strip()
 
 
-def generate_name_node(name_ids, ctx=gast.Load()):
+def generate_name_node(name_ids, ctx=gast.Load(), gen_tuple_if_single=False):
     """
-    Generate list or gast.Tuple of ast.Name for Return statement.
+    If name_ids is list or tuple or set with multiple strings, this function
+    generates gast.Tuple of gast.Name.
+    If the name_ids is single string or contains only 1 string, this function
+    returns gast.Name if gen_tuple_if_single==False else returns gast.Tuple
+    with only one gast.Name
+
+    This function is used at several gast.Return statements.
     """
     if isinstance(name_ids, six.string_types):
         name_ids = [name_ids]
@@ -395,7 +401,7 @@ def generate_name_node(name_ids, ctx=gast.Load()):
             id=name_id, ctx=ctx, annotation=None, type_comment=None)
         for name_id in name_ids
     ]
-    if len(gast_names) == 1:
+    if len(gast_names) == 1 and not gen_tuple_if_single:
         name_node = gast_names[0]
     else:
         name_node = gast.Tuple(elts=gast_names, ctx=ctx)

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

@@ -233,6 +233,7 @@ def for_iter_var_idx(x_array):
     return z
 
 
+# 17. for a,b,c in z: (a, b, c) is a tuple
 @paddle.jit.to_static
 def for_tuple_as_iter_var(x_array):
     x = paddle.to_tensor(x_array)
@@ -250,6 +251,7 @@ def for_tuple_as_iter_var(x_array):
     return a_result, b_result, c_result
 
 
+# 18. for t in enumerate(collection): t is tuple of (idx, element)
 @paddle.jit.to_static
 def for_tuple_as_enumerate_iter(x_array):
     x = paddle.to_tensor(x_array)
@@ -263,6 +265,7 @@ def for_tuple_as_enumerate_iter(x_array):
     return a_result
 
 
+# 19. for i, (a, b, c, d, e) in enumerate(collection): (a, b, c, d, e) is a tuple
 @paddle.jit.to_static
 def for_tuple_as_enumerate_value(x_array):
     x = paddle.to_tensor(x_array)
@@ -284,6 +287,23 @@ def for_tuple_as_enumerate_value(x_array):
     return a_result
 
 
+# 20. test for function in a class
+class ForwardContainsForLayer(paddle.nn.Layer):
+    def __init__(self):
+        super(ForwardContainsForLayer, self).__init__()
+        self.high = 5
+        self.low = 3
+
+    @paddle.jit.to_static
+    def forward(self, x):
+        # just for test case, x is useless in this method
+        y = paddle.zeros([10, 2, 3])
+        z = []
+        for i in range(self.high - self.low):
+            z.append(y[i].clone())
+        return z
+
+
 class TestTransformBase(unittest.TestCase):
     def setUp(self):
         self.place = fluid.CUDAPlace(0) if fluid.is_compiled_with_cuda(
@@ -313,11 +333,11 @@ class TestTransformBase(unittest.TestCase):
 class TestTransform(TestTransformBase):
     def transformed_result_compare(self):
         dy_outs = self.get_dygraph_output()
-        if not isinstance(dy_outs, tuple):
+        if not isinstance(dy_outs, (tuple, list)):
             dy_outs = (dy_outs, )
 
         st_outs = self.get_static_output()
-        if not isinstance(st_outs, tuple):
+        if not isinstance(st_outs, (tuple, list)):
             st_outs = (st_outs, )
 
         for x, y in zip(dy_outs, st_outs):
@@ -446,5 +466,10 @@ class TestForTupleAsEnumerateValue(TestForIterVarNumpy):
         self.dygraph_func = for_tuple_as_enumerate_value
 
 
+class TestForwardContainsForLayer(TestForIterVarNumpy):
+    def set_test_func(self):
+        self.dygraph_func = ForwardContainsForLayer()
+
+
 if __name__ == '__main__':
     unittest.main()