[pls merge] rearrange examples (#802)

* arrange examples

* reinforce file

* add slow

* add readme

* change online docs

* yapf

* remove images

* yapf

* fix docstyle

* don't use import *

* example --> examples

* docs example --> examples, tutorial --> tutorials

* fix yapf

* add changelog

* restore conflict vgg19

* readme

* rm example folder

* remove applications

* fix yapf -cost.py

* rm applications

* yapf

Author: zsdonghao