* starting to merge fsaf

* changed configs

* changed bbox_coder

* get_targets fun in AnchorHead class is modified to enable additional returns from _get_target_single

* effective area assigner and tblr bbox_coder are implemented

* added fsaf_head and config

* added more fsaf configs

* fixed the nan bbox_loss by clamping bbox_pred with min=1e-4

* fixed the nan bbox_loss by clamping bbox_pred with min=1e-4

* changed config

* formatter

* changed docstring

* Added ignored pixel-class pair for FSAF

* More elegant assignment for overlapping objects

* more documents in effective area assigner

* fixed a bug in object overlay

* Added no gt_bboxes case and pytest for assigner

* Added no gt_bboxes case and pytest for assigner

* changed default config

* fixed small bugs in FSAF head doc example

* resolved pr review comments.
1. Formats of comments and docstrings are modified
2. Obsolete arguments are removed from FSAFHead
3. AssignResults class is revised to be more compatible with old version

* Added more comments to FSAFHead

* resolved pr review comments

* resolved pr review comments
1. More flexible CenterRegionAssigner bbox priority
2. More flexible tblr_coder by adding normalize_by_wh flag
3. README modifications with model link
4. More docstrings and comments
5. Reformatted
6. Change of default normalizer in TBLRBBoxCoder from 1.0 to 4.0

* Corrected FSAF model path in readme

* Fixed the bug of bbox weights in fsaf head

Co-authored-by: wangxinjiang <wangxinjiang@sensetime.com>