For a few datasets that detectron2 natively supports, the datasets are assumed to exist in a directory called "datasets/", under the directory where you launch the program. They need to have the following directory structure:

cityscapes/
  gtFine/
    train/
      aachen/
        color.png, instanceIds.png, labelIds.png, polygons.json,
        labelTrainIds.png
      ...
    val/
    test/
  leftImg8bit/
    train/
    val/
    test/

Install cityscapes scripts by:

pip install git+https://github.com/mcordts/cityscapesScripts.git

Note: labelTrainIds.png are created by cityscapesscripts/preparation/createTrainIdLabelImgs.py. They are not needed for instance segmentation.

VOC20{07,12}/
  Annotations/
  ImageSets/
  JPEGImages/