[New model] Support CGNet (#223)

* added cgnet

* added testing for cgnet

* git test

* add cgnet

* fix __init__

* rename FGlo with GlobalContextExtractor

* add readme.md and rename bn with norm

* delete cg_head

* fix a language mistake

* rename cgnet_m3n21.py to cgnet.py

* modify README.md

* modify list to tuple

* add fcn_head test

* add assert to fcn_head

* blank

* fix fcn_head assert bug

* add

* add cgnet to README.md and model_zoo.md

* modify cgnet README.md

Co-authored-by: KID <wps_@mail.nankai.edu.cn>