Add SemanticKITTI LiDAR segmentation tutorial (#590)

* Add Citscapes dataset adapter

* Add Cityscapes dataset adapter and cleanup

* Update datasets/__init__.py

* Correction in build_dataset arg

* Add tutorial for cityscapes semanticsegmentation

* Restore cityscacpescripts dependency

* Add SemanticKITTI dataset adapter

* Add config builder functions

* Add SemanticKITTI LiDAR segmentation tutorial

* Black formatting

* Restore datasets/__init__.py

* Add sparseconv dependency detail

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Co-authored-by: David Pascual-Hernández <d.pascualhe@gmail.com>

Author: tejasstanley