Note that this PR is a modified version of the withdrawn PR
https://github.com/open-mmlab/mmsegmentation/pull/1748

## Motivation

In the last years, panoptic segmentation has become more into the focus
in reseach. Weber et al.
[[Link]](http://www.cvlibs.net/publications/Weber2021NEURIPSDATA.pdf)
have published a quite nice dataset, which is in the same style like
Cityscapes, but for KITTI sequences. Since Cityscapes and KITTI-STEP
share the same classes and also a comparable domain (dashcam view),
interesting investigations, e.g. about relations in the domain e.t.c.
can be done.

Note that KITTI-STEP provices panoptic segmentation annotations which
are out of scope for mmsegmentation.

## Modification

Mostly, I added the new dataset and dataset preparation file. To
simplify the first usage of the new dataset, I also added configs for
the dataset, segformer and deeplabv3plus.

## BC-breaking (Optional)

No BC-breaking

## Use cases (Optional)

Researchers want to test their new methods, e.g. for interpretable AI in
the context of semantic segmentation. They want to show, that their
method is reproducible on comparable datasets. Thus, they can compare
Cityscapes and KITTI-STEP.

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Co-authored-by: CSH <40987381+csatsurnh@users.noreply.github.com>
Co-authored-by: csatsurnh <cshan1995@126.com>
Co-authored-by: 谢昕辰 <xiexinch@outlook.com>