[Enhancement] Delete convert function and add instruction to ViT/Swin README.md (#791)

* delete convert function and add instruction to README.md

* unified model convert and README

* remove url

* fix import error

* fix unittest

* rename pretrain

* rename vit and deit pretrain

* Update upernet_deit-b16_512x512_160k_ade20k.py

* Update upernet_deit-b16_512x512_80k_ade20k.py

* Update upernet_deit-b16_ln_mln_512x512_160k_ade20k.py

* Update upernet_deit-b16_mln_512x512_160k_ade20k.py

* Update upernet_deit-s16_512x512_160k_ade20k.py

* Update upernet_deit-s16_512x512_80k_ade20k.py

* Update upernet_deit-s16_ln_mln_512x512_160k_ade20k.py

* Update upernet_deit-s16_mln_512x512_160k_ade20k.py

Co-authored-by: Jiarui XU <xvjiarui0826@gmail.com>
Co-authored-by: Junjun2016 <hejunjun@sjtu.edu.cn>