Refactor hungarian assigner (#4259)

* add focal_loss in hungarian_assigner.py

* update hungarian_assigner.py

* update detr config

* update doc and codes

* fix unitest

* fix unitest

* modify code format

* update docstring

* fix unitest

* fix name

* fix docformat

* fix docformat

* fix format

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