* Added dygraph quantization.

* Added save_quant_model function

* Overload quant_aware, make dynamic and static model use the same interface.

* Use singledispatch to overload the quant aware for dygraph quantization

* Add unnitest for dygraph quant_aware

* Use 2.0 api for supporting paddle 2.0

* Using pre-commit to modify the coding style.

* Add unittest for user inputted quant_config.

* Add singledispath into requirements.txt
Co-authored-by: NBai Yifan <me@ethanbai.com>