Created by: qingqing01

1. rename: PaddleVideo -> video, PaddleGAN -> gan, Paddle3D -> 3d_vision
2. remove models/PaddleSlim
3. add PaddleDetection as submodules.
4. Update link in models/README.md and models/PaddleCV/README.md

see https://github.com/qingqing01/models/tree/release/1.7-2/PaddleCV