VarifocalNet (#3666)

* add varifocal loss, vfnet head and config file

* fix nonzero() and register_module bug

* add more configs and README.md

* enlarge image in README.md

* add arxiv link

* add paper arxiv link

* attempt to fix checks error - res2net

* update readme

* update according to comments

* revert res2net.py

* compact the VFNetHead

* adapt for official TTA

* add unit tests and refine vfnet_head

* update url

* update readme

* add minor comments

* update supported models

* remove VFNet.png

Co-authored-by: Cao Yuhang <yhcao6@gmail.com>