* cherry-pick #1406,#1407,#1420,#1422

minor grammatical corrections
Co-authored-by: Nqingqing01 <dangqingqing@baidu.com>

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

add fleet training

* fix voc dataset and docs

* add custom datsset demo

Add image for visualdl board.

* fix some docs link error

* rename config_doc

* fix export_model doc link error

* sort out release/0.3 docs

* add new CHANGELOG

* add yolov4 in changelog

* Update docs
* Update README.md

* optimize python path

* Remove `pycocotools` from `requirements.txt`

git version is required for numpy > 1.18

* Remove `cython`, add pip install doc

* add config link in model zoo

* fix link error

* add anchor free

* refine transfer learn

* refine transfer learning doc

* fix fruit config on master branch

* refine doc

* fix all documentation

* fix docs conf.py

* fix some error

* fix readthedocs