* add esrgan model, refine codes

* docs: update install instruction

1. paddlepaddle2.0.0rc instruction can be found from main page
2. only two steps, install paddlepaddle and paddlegan, other than 3step

* docs: update en install
* feat: add dcgan
* reform: debug with mnist datset
validate performance with different options, and original structure and setting

* refine codes docs

* fix pix2pix to_tensor

* fix apps

* add art model of deoldify

* Add doc for Apps and refine some code
* Add how to switch cpu and gpu

* refine code

* test

* fix apps

* update readme

* rm unused code

* fix apps output when input is image

* clean code

* update requirements.txt