Created by: liym27

• PR types: New features

• PR changes: dygraph to static graph

• Describe:

1.Remove after_visit in IfElseTransformer, and use simpler code to achieve the same effect.

2. Support return variable in only one of If.body or If.orelse node.

E.g.

x, y = 5, 10
if x > 4:
    x = x+1
    z = x*x
    q = 10
else:
    y = y - 1
    z = y*y
    m = 20
    n = 20

print(q)
n = 30
print(n)

The return_ids are (x, y, z, q) for If.body and If.orelsenode, because

1. x is modified in If.body node,
2. y is modified in If.body node,
3. z is both created in If.body and If.orelse node,
4. q is created only in If.body, and it is used by print(q) as gast.Load. Note: After transformed, q is created in parent scope. For example,

x, y = 5, 10
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_func(x, y, q):
    x = x+1
    z = x*x
    q = 10
    return x,y,z,q

def false_func(x, y, q):
    y = y - 1
    z = y*y
    m = 20
    n = 20
    return x,y,z,q
x,y,z,q = fluid.layers.cond(x>4, lambda: true_func(x, y), lambda: false_func(x, y, q))

m and n are not in return_ids, because 5. m is created only in If.orelse, but it is not used after gast.If node. 6. n is created only in If.orelse, and it is used by n = 30 and print(n), but it is not used as gast.Load firstly but gast.Store .