* update 2.1 readme

* update en doc, test=document_fix

* update doc, test=document_fix
Co-authored-by: NGuanghua Yu <742925032@qq.com>

* update slim models and benchmark

* support no label training

* fix load_crowd

* update to fit rbox

* add empty_ratio

* update configs

* support mask rcnn

* clean drop_empty for mot & keypoint

* refine is_crowd

* fix ffmpeg video infer, fix doc, test=document_fix

* fix ffmpeg installl, test=document_fix

* Add document about how to export ONNX model

* add model_zoo unittest

* update det infer benchmark log

* remove cython_bbox, test=document_fix

* add cython_bbox install log, test=document_fix

* fix mot_source

* fix mot_source, test=document_fix

* fix jde negative coords

* update jde model zoo

* describe format for some txt, test=document_fix

* fix mot doc

* remove image_lists, fix all mot docs, add custom data

* fix doc, test=document_fix

* fix dov

* fix doc format, test=document_fix

* fix for_mot in PPYOLOTinyFPN and PPYOLOPAN

* fix yolo_fpn

* add video infer for TestMOTReader

* format, test=documment_fix

* update install docs

* fix slim doc,test=document_fix

* fix config dependence & fix python 3.8/3.9 install

* Update README.md

* add fairmot

* rename pretrain_weights

* add comments; reuse focal loss and convnorm

* update comments

* fix hsv randomaffine

* fix nms return_index

* fix gt_ide by tid_start_index