Add post_process_semantic_segmentation method to SegFormer
# What does this PR do?
Adds a post_process_semantic_segmentation method to `SegFormerFeatureExtractor` with optional resizing. This model doesn't support instance or panoptic segmentation. 

I will open separate PRs to make sure the naming and outputs of post_process methods of segmentation models are consistent.

## 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](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [ ] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? 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](https://github.com/huggingface/transformers/tree/main/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


_The documentation is not available anymore as the PR was closed or merged._<|||||>@NielsRogge  FYI, I will also need to open a PR to edit `ImageSegmentationPipeline` after making sure all post-processing methods across segmentation models are consistent in terms of naming and functionality.<|||||>Hey @alaradirik, please also ping a core maintainer for review before merging PRs.