[Feature] add setr cityscapes benchmark (#1087)

* [Feature] add setr cityscapes benchmark

* change pretrain

* Update configs/_base_/datasets/cityscapes_768x768.py

Co-authored-by: Junjun2016 <hejunjun@sjtu.edu.cn>

* remove redundant keys

* remove redundant keys

* fix lint error

* update readme

* update pretrain

Co-authored-by: Junjun2016 <hejunjun@sjtu.edu.cn>