Created by: lcy-seso

GAN is one of some of the old examples that haven't merged into PaddlePaddle yet. Here is the original pull request: https://github.com/PaddlePaddle/book/pull/30. We are going to merge it into PaddlePaddle first.

GAN will be a good example to enhance PaddlePaddle's control flow of sub-graphs in the entire network during training.