Add BBoxAssigner and BBoxSampler components for better modular usage

Bug fix for compiling nms op

Update citation bibtex

Update readme with some training notes

 bug fix in coco dataset

update coco dataset to support proposals with shapes (n, 5)

Add some instructions of customizing models

Initial public release

Add documentation

Update inference APIs

Rename the subpackage api to apis

add high level api

minor fix for fast rcnn