* add centertrack base codes

* fix deploy and docs

* fix tracker

* fix

* fix pre_img

* fix deploy

* fix

* update version and changelog for release/2.4

* update README

* update doc, test=document_fix

* add citation

* update citation and ppyolo doc

* add docs for custom dataset, reader and model technical

* modify docs according to review, test=document_fix

* modify READER.md, test=document_fix

* add benchmark, test=document_fix

* update style of number, test=document_fix

* add difference of dynamic shape and fix shape

* add pedestrian and vehicle models

* add pedestrian and vehicle json

* modify pedestrian and vehicle configs

* modify docs and links, test=dygraph

* fix_quickstart_doc

* fix model_zoo package for configs directory structure change

* update README

* fix modelzoo link

* fix config, remove some faster mask modelzoo

* fix readme

* fcos infer

* fcos training

* fix stgraph fcosloss

* fcos doc

* fix dy_fcos

* add comment

* train eval deploy fcos

* remove fcos config, add comment

* fix iter_tic
Co-authored-by: Nnemonameless <nemonameless@qq.com>
Co-authored-by: Ngeorgenemo <nifeng@pku.edu.cn>

* add solov2 dygraph model, test=dygraph

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>