Support FP16 training (#520)

* add fp16 support

* fpn donot need bn normalize

* refactor wrapped bn

* fix bug of retinanet

* add fp16 ssd300 voc, cascade r50, cascade mask r50

* fix bug in cascade rcnn testing

* add support to fix bn training

* add fix bn cfg

* delete fixbn cfg, mv fixbn fp16 to a new branch

* fix cascade mask fp16 bug in test

* fix bug in cascade mask rcnn fp16 test

* add more fp16 cfgs

* add fp16 fast-r50 and faster-dconv-r50

* add fp16 test, minor fix

* clean code

* fix config work_dir name

* add patch func, refactor code

* fix format

* clean code

* move convert rois to single_level_extractor

* fix bug for cascade mask, the seg mask is ndarray

* refactor code, add two decorator force_fp32 and auto_fp16

* add fp16_enable attribute

* add more comment, fix format and test assertion

* fix pep8 format error

* format commont and api

* rename distribute to distributed, fix dict copy

* rename function name

* move function, add comment

* remove unused parameter

* mv decorators into decorators.py, hook related functions to hook

* add auto_fp16 to forward of semantic head

* add auto_fp16 to all heads and fpn

* add docstrings and minor bug fix

* simple refactoring

* bug fix for patching forward method

* roi extractor in fp32 mode

* fix flake8 error

* fix ci error

* add fp16 support to ga head

* remove parallel test assert

* minor fix

* add comment in build_optimizer

* fix typo in comment

* fix typo enable --> enabling

* udpate README