Add semantic segmentation post-processing method to MobileViT

[alaradirik]

What does this PR do?

Adds post_process_semantic_segmentation method to MobileViTFeatureExtractor.

I will open an issue and separate PRs to make sure that

• Segmentation models (DETR, MaskFormer, SegFormer, etc.) have consistently named post-processing methods, arguments and outputs
• ImageSegmentationPipeline works with all available segmentation models

Before submitting

• [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• [ X] Did you read the contributor guideline, Pull Request section?
• [ ] Was this discussed/approved via a Github issue or the forum? Please add a link to it if that's the case.
• [ X] Did you make sure to update the documentation with your changes? Here are the documentation guidelines, and here are tips on formatting docstrings.
• [ ] Did you write any new necessary tests?

[HuggingFaceDocBuilderDev]

The documentation is not available anymore as the PR was closed or merged.