Created by: littletomatodonkey

paddle1.7版本，py_func中在输出out为list时，没有做处理，导致out_list未被定义，程序报错

https://github.com/PaddlePaddle/Paddle/blob/e78858f19c9d52d22431b453cb0e776e4c98ef99/python/paddle/fluid/layers/nn.py#L12599