* cherry-pick #2234, test=dygraph

* add gn head fpn, test=dygraph

* add gn for cascade

* update gn readme, test=dygraph

* from_config hrnet, test=dygraph

* update hrnet modelzoo, test=dygraph

* fix bbox_num 0 after nms, test=dygraph

* add copyright, test=dygraph

* fix RCNN dygraph to static

* fix problem due to empty bbox_pred and modify ppyolo docs, test=dygraph

* modify post process to avoid dy2st problem

* add quickstart doc, roadsign dataset

* add dygraph quant

* add ppyolo r18vd mbv3, yolov3 r50vd

* modify TestReader of ppyolo mbv3 r18vd

* add clip to avoid nan, modify ema to apply ema on bn mean and bn var

* fix code resulting in nan

* add yolov3 r50vd dcn configs

* finish yolo_series and fix some problems

* hide --bias flag and modify docs

* modify --bias and fix deploy/python/infer

* fix load_weight and no bbox output

* fit ttfnet

* fit for YOLOvd/PPYOLO/SSD

* eval in train

* add prune dygraph, test=dygraph

* fix slim readme and prune

* refactor rcnn, test=dygraph

* add mask_rcnn, test=dygraph

* add Faster RCNN & Faster FPN, test=dygraph

* update according to the review, test=dygraph

* fix modelzoo link

* fix config, remove some faster mask modelzoo

* fix readme

* fix reader exit error
* fix commet

* fix ssd mbv1 config

* fix trt infer