* YOLACT (not the final commit, so please don't review)

* Implementation notes for YOLACT

* Modify yolact_r50.py

* Update YOLACT configs

* Add YOLACT README

* Adjust according to the first round of review

* Handle the corner case when gt_mask is empty

* Add unit tests for YOLACT heads

* Add unit tests for YOLACT forward

* Adjust according to Kai's comments, except for gt_mask filtering

* Change use_OHEM to use_ohem in unit tests

* Update config in YOLACT unit tests

* merge bbox overlap and batch bbox overlap

* fix unit test

* refactor some codes and fix some bugs

* fix isort

* fix ci

* update model link

* fix cfg path

* support batch test

* Make get_seg_masks faster

* fix format

* update readme

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