[Dy2Static]Support return variable created in only one of If.body or If.orelse (#24841)

* Support return variable in only one of if body or else. 

* remove after_visit in IfElseTransformer.

* Modify the result of get_name_ids in test_ifelse_basic.py 

* Add unittest to test the new case. 

* Modify code according to reviews.

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

@@ -52,6 +52,51 @@ def dyfunc_with_if_else2(x, col=100):
     return y
 
 
+def dyfunc_with_if_else3(x):
+    # Create new var in parent scope, return it in true_fn and false_fn.
+    # The var is created only in one of If.body or If.orelse node, and it used as gast.Load firstly after gast.If node.
+    # The transformed code:
+    """
+    q = fluid.dygraph.dygraph_to_static.variable_trans_func.
+        data_layer_not_check(name='q', shape=[-1], dtype='float32')
+    z = fluid.dygraph.dygraph_to_static.variable_trans_func.
+            data_layer_not_check(name='z', shape=[-1], dtype='float32')
+
+    def true_fn_0(q, x, y):
+        x = x + 1
+        z = x + 2
+        q = x + 3
+        return q, x, y, z
+
+    def false_fn_0(q, x, y):
+        y = y + 1
+        z = x - 2
+        m = x + 2
+        n = x + 3
+        return q, x, y, z
+    q, x, y, z = fluid.layers.cond(fluid.layers.mean(x)[0] < 5, lambda :
+        fluid.dygraph.dygraph_to_static.convert_call(true_fn_0)(q, x, y),
+        lambda : fluid.dygraph.dygraph_to_static.convert_call(false_fn_0)(q,
+        x, y))
+    """
+    y = x + 1
+    # NOTE: x_v[0] < 5 is True
+    if fluid.layers.mean(x).numpy()[0] < 5:
+        x = x + 1
+        z = x + 2
+        q = x + 3
+    else:
+        y = y + 1
+        z = x - 2
+        m = x + 2
+        n = x + 3
+
+    q = q + 1
+    n = q + 2
+    x = n
+    return x
+
+
 def nested_if_else(x_v):
     batch_size = 16
     feat_size = x_v.shape[-1]

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

@@ -64,18 +64,24 @@ class TestDygraphIfElse2(TestDygraphIfElse):
 
 
 class TestDygraphIfElse3(TestDygraphIfElse):
+    def setUp(self):
+        self.x = np.random.random([10, 16]).astype('float32')
+        self.dyfunc = dyfunc_with_if_else3
+
+
+class TestDygraphNestedIfElse(TestDygraphIfElse):
     def setUp(self):
         self.x = np.random.random([10, 16]).astype('float32')
         self.dyfunc = nested_if_else
 
 
-class TestDygraphIfElse4(TestDygraphIfElse):
+class TestDygraphNestedIfElse2(TestDygraphIfElse):
     def setUp(self):
         self.x = np.random.random([10, 16]).astype('float32')
         self.dyfunc = nested_if_else_2
 
 
-class TestDygraphIfElse5(TestDygraphIfElse):
+class TestDygraphNestedIfElse3(TestDygraphIfElse):
     def setUp(self):
         self.x = np.random.random([10, 16]).astype('float32')
         self.dyfunc = nested_if_else_3

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

@@ -34,7 +34,7 @@ class TestGetNameIds(unittest.TestCase):
           def test_fn(x):
             return x+1
         """
-        self.all_name_ids = {'x': [gast.Param()]}
+        self.all_name_ids = {'x': [gast.Param(), gast.Load()]}
 
     def test_get_name_ids(self):
         source = textwrap.dedent(self.source)
@@ -82,6 +82,7 @@ class TestGetNameIds2(TestGetNameIds):
                 gast.Load(),
                 gast.Store(),
                 gast.Store(),
+                gast.Load(),
             ]
         }
 
@@ -113,6 +114,7 @@ class TestGetNameIds3(TestGetNameIds):
                 gast.Store(),
                 gast.Load(),
                 gast.Store(),
+                gast.Load(),
             ]
         }