Fix API equation for img_pool and maxout format
Created by: chengduoZH
Currently, the format of equation for img_pool and maxout is very messy. paddle.v2.layer.img_pool paddle.v2.layer.maxout
Created by: chengduoZH
Currently, the format of equation for img_pool and maxout is very messy. paddle.v2.layer.img_pool paddle.v2.layer.maxout