* add ppyolov2

* fix bugs and modify docs

* modify code and doc according to review

* fix bugs while resolving conflicts

* update README

* update doc, test=document_fix

* add citation

* update citation and ppyolo doc

* add PP-YOLO tiny

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

* modify post process to avoid dy2st problem

* 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

* add ppyolo in dygraph

* fix bugs

* modify configs

* polish docs, modify ema and add configs

* modify link of model

* fix bugs in ema

* modify code according to review

* modify minicoco configs