Add cityscapes dataset (#1037)

* added cityscapes

* updated configs

* removed wip configs

* Add initial dataset instructions

* Add cityscapes readme

* Add explanation for lr scaling

* Ensure pep8 conformity

* Add CityscapesDataset to the registry

* add benchmark

* rename config, modify README.md

* fix typo

* fix typo in config

* modify INSTALL.md

Update information how to arrange cityscapes data.

* Add cityscapes class names