Improve semantic segmentation models (#14355)
* Improve tests
* Improve documentation
* Add ignore_index attribute
* Add semantic_ignore_index to BEiT model
* Add segmentation maps argument to BEiTFeatureExtractor
* Simplify SegformerFeatureExtractor and corresponding tests
* Improve tests
* Apply suggestions from code review
* Minor docs improvements
* Streamline segmentation map tests of SegFormer and BEiT
* Improve reduce_labels docs and test
* Fix code quality
* Fix code quality again
