reimplement cityscapes (#2089)

* reimplement cityscapes

* fixed gt bbox mode

* convert cityscapes to coco style, add cityscapes eval

* add cityscapes convert script

* add doc

* Update INSTALL.md

* Update INSTALL.md

* update fater rcnn

* fix cityscapes eval

* support format only in cityscapes

* add docs

* remove redundancy

* resolve eval

* update cityscapes md

* more doc and rename

* update doc and cfg

* change to test set