Co-authored-by: NFeng Ni <nemonameless@qq.com>

* add ttfnet enhance

* add doc

* fix pafnet training

* add ppyolov2

* fix bugs and modify docs

* modify code and doc according to review

* fix bugs while resolving conflicts

* try fix travis

* add PP-YOLO tiny

* add s2anet

* add pssdet model

* fix name of pretrain weights

* format, test=document_fix

* fix warning

* add senet

* add annotations according to review

* add Distill model

* add distill+prune

* add docs for custom dataset, reader and model technical

* modify docs according to review, test=document_fix

* modify READER.md, test=document_fix

* refine ppdet file name

* unify bbox_utils

* update files

* update for ci-travis

Co-authored-by: NGuanghua Yu <742925032@qq.com>
Co-authored-by: NFeng Ni <nemonameless@qq.com>

* refine trainer

* update install doc, test=document_fix

* update doc, test=document_fix