Co-authored-by: NFeng Ni <nemonameless@qq.com>

* add ttfnet enhance

* add doc

* fix pafnet training

* add s2anet

* refine trainer

* update install doc, test=document_fix

* update doc, test=document_fix

add htc model 

* filter illegal data

* update warning message

* add yolov4

* Make logger less noisy

* Fix indent

* fix all documentation

* fix docs conf.py

* fix some error

* fix readthedocs

* refine cocoapi import

* fix infer when anno_path is None

* Polish Reader to simplify preprocessing logic.
#  sample_transforms-> make batch -> batch_transforms in Reader.
* Clean some code
* Imporve yolov3_r50vd_dcn_obj365_pretrained_coco 41.4 to 41.8.
* Update all configs.

Fix hang bug in parallel_map.py when multiprocessing mode used and some consumer exit abnormally (#106)

* fix mixup error when dataset has only 1 sample

* refine log

* update code & doc

* change voc_loader

* add class aware sampling strategy
* remove redundancy code

* refine assert info in roidb_source.py

* refine image_dir assert

* Add backbone and architecture for FaceBoxes and Blazeface.
* Add dataloader for WIDERFace dataset.

* add a global norm_type flag

* add group norm on fpn

* add gn on box head

* add faster rcnn fpn 50 gn config

* add mask branch norm

* update configs

* standard SENet and add a warning in coco loader