* update cascade, test=dygraph

* clean code, test=dygraph

* add cascade_rcnn, test=dygraph

* update according to reviews, test=dygraph

* support dy2static, test=dygraph

* refactor rcnn, test=dygraph

* add mask_rcnn, test=dygraph

* add Faster RCNN & Faster FPN, test=dygraph

* update according to the review, test=dygraph

* add solov2 dygraph model, test=dygraph

* add mbv1v3-yolov3

* add ssd mbv1v3 voc

* yolov3 mbv1v3 voc

* training log fix

* yolov3 voc fix

* yolov3 mbv1v3 training and eval, ssd fix

* add syncbn

* mbv1v3 clean code

* mbv1v3 clean codes

* remove ssd mbv1v3

* update modelzoo doc

* remove others

* fix ssd reader and prior_box

* ssd mbv1 config

* fix anchor steps and reader op

* ssdlite mbv1v3 infer and train

* update yolov3 config and modelzoo

* update yolov3_mbv3 modelzoo

* pre commit update

* update modelzoo

* fix mbv3 in yolov3

* fix precommit
Co-authored-by: Nnemonameless <nemonameless@qq.com>

Merge dygraph branch into master branch.
Co-authored-by: NFDInSky <48318485+FDInSky@users.noreply.github.com>
Co-authored-by: Nsunxl1988 <47514455+sunxl1988@users.noreply.github.com>
Co-authored-by: Nwangguanzhong <jerrywgz@126.com>
Co-authored-by: Nwangxinxin08 <69842442+wangxinxin08@users.noreply.github.com>
Co-authored-by: Nwanghuancoder <wanghuancoder@163.com>
Co-authored-by: Ncnn <liuhui29@baidu.com>
Co-authored-by: NGuanghua Yu <742925032@qq.com>
Co-authored-by: NKaipeng Deng <dengkaipeng@baidu.com>
Co-authored-by: Nroot <root@bjyz-sys-gpu-kongming0.bjyz.baidu.com>