Created by: chengduoZH
Add pool op.
- Add pooling2d max and average (gpu and cpu implementations)
- Add pooling3d max and average (gpu and cpu implementations)
- Add unit test pooling2d and pooling3d
fixed https://github.com/PaddlePaddle/Paddle/issues/4231 fixed https://github.com/PaddlePaddle/Paddle/issues/3692 fixed https://github.com/PaddlePaddle/Paddle/issues/4165 fixed https://github.com/PaddlePaddle/Paddle/issues/4164