* add cascade mask

* add cascade rcnn

* update code

* remove comments

* add faster_rcnn_r50_fpn and faster_rcnn_r50

* update TestReader in dygraph

* if gt_ploy is empty, continue

* fix typo

* judge gt_ploy is empty or not

* fix judge condition

* fix gt_ploy judge

* split config into models, optimizers, dataset, readers，runtime

* refine PaddleDetection benchmard print, test=develop

* refine PaddleDetection benchmard print, test=develop

* refine PaddleDetection benchmard print, test=develop

* refine PaddleDetection benchmard print, test=develop

* refactor dygraph & add mask rcnn fpn

* fix initialization & minor update

* refine architecture of mask_rcnn

add train&eval 
add reader padding 
add new config 

clear directories & files for dygraph branch 

* 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.

* 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

* fix infer doc and config default

* refine dataset exists

* fix None proc

* Clean up a bit

* Add `__shared__` annotation for shared variables between modules

* Dedup `num_classes` configurations

* Document `__shared__` usage

* Remove some unused variable

* Improve docstring and add comments for `__shared__`

* Rename object_detection to PaddleDetection
* Small fix for doc