Code for CVPR 2019 Paper "Libra R-CNN: Towards Balanced Learning for Object Detection" (#687)

* Code for components of Libra R-CNN

* Configs and README for Libra R-CNN

* update bfp

* Update Model ZOO

* add comments in non-local

* fix shape

* update bfp

* update according to ck's comments

* update des

* update des

* fix loss

* fix according to ck's comments

* fix activation in non-local

* fix conv_mask in non-local

* fix conv_mask in non-local

* Remove outdated model urls

* refactoring for bfp

* change in_channels from list[int] to int

* refactoring for nonlocal

* udpate weight init of nonlocal

* minor fix

* update new model urls