GFocal (#3097)

* add gfocal losses

* add gfl head, its related code, configs

* fix 2x config bug

* add most model and log in gfl README

* update gfl model and log

* Update transforms.py

modify variables  L, T, R, B   to  left, top, right, bottom

* Update gfocal_loss.py

modify variables into meaningful names:
* pt -> scale_factor
* a -> pos   b -> pos_label
* disl -> dis_left  disr -> dis_right
* wl -> weight_left  wr -> weight_right

* Update gfocal_loss.py

update formats

* remove duplicated get_bboxes/IoU calculation, make QFL consistent

* add all docstring in gfl_head.py and gfocal_loss.py