* 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

* Fix ResizeImage and data transform for SSD
* Fix code style

* Rename object_detection to PaddleDetection
* Small fix for doc