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

* add ttfnet enhance

* add doc

* fix pafnet training

* update README

* update doc, test=document_fix

* add citation

* update citation and ppyolo doc

* update readme en, test=document_fix

* update doc

* add 2.0 CHANGLOG

* add changelog history

* update doc

* notes for static -> dygraph

* move ssld model, test=document_fix

* fix typo, test=document_fix

* 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

* add static notice

* fix warning

* add senet

* add annotations according to review

* update slim model

* fix slim readme

* 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

* [doc] add doc of benchmark (#2505)

* add benchmark, test=document_fix

* update style of number, test=document_fix

* add difference of dynamic shape and fix shape

* update for ci-travis, test=document_fix

* update files

* update for ci-travis