Reimplement "FreeAnchor: Learning to Match Anchors for Visual Object Detection" (#1391)

* add FreeAnchorRetinanet

* fix std

* refactor code

* add cfg

* fix bug in cfg

* add annotation and clean

* add readme

* fix isort error

* replace clip with torch.clamp

* fix init typo

* refactoring

* clean

* update benchmark

* rename and delete some var

* minor fix