* add cascade gn

* add cascade mask gn config

* update configs

* fix train on VOC on multi-GPU hang

* not set persistable in parse_fetches

* fix rpn_only in cascade mask fpn

* fix rpn_only in cascade rcnn

* add gn doc

* refine cascade models

* update model zoo doc

* cascade-mask-fpn add

* 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

* add yolov3_darknet cinfig comment

* fix commment

* add log_iter

* Add full configs example & refine doc

* refine docs

* add_ssd_config_demo

* add_license for PaddleCV

* update license date

* update GAN license date

* Add cascade deformable mdoels
* Fix MODLE_ZOO.md

# Add VGG-SSD on VOC and COCO dataset.
# Add config and model zoo. 
# Refine bbox2out and draw_bbox:
    - Add bbox de-normalize in bbox2out function.
    - Remove bbox de-normalize in draw_bbox.

* update doc in PaddleDetection

* refine weight download

* fix comment

* add save best model & fix dcn configs

* fix yolov3 voc eval in train

* fix comment

* standard SENet and add a warning in coco loader

* add deformable models
* fix dcn v2
* fix log_iter

* fix cn docs

* fix easy problem

* update GETTING_STARTED.md