Code for AAAI 2021 paper. "SCNet: Training Inference Sample Consistency for...

Code for AAAI 2021 paper. "SCNet: Training Inference Sample Consistency for Instance Segmentation" (#4356)

* add scnet

* add more configs and minors

* fix unittest for scnet

* update docstring

* add forward test for htc and scnet

* fix build on pytorch 1.3 for empty tensor

* update doc string and minor refactor

* update unit-test and use inheritance for SCNet heads

* support scnet tta

* minor

* update readme

* minor fixes