Improve and fix ImageSegmentationPipeline
# What does this PR do?
- Fixes the image segmentation pipeline test failures caused by changes to the postprocessing methods of supported models
- Updates the ImageSegmentationPipeline tests
- Improves docs, adds 'task' argument to optionally perform semantic, instance or panoptic segmentation

Note: `test_small_model_pt` test is skipped due to a random weight initialization error when loading the `hf-internal-testing/tiny-detr-mobilenetsv3-panoptic` model.

## 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).
- [ X] Did you write any new necessary tests?

_The documentation is not available anymore as the PR was closed or merged._<|||||>@amyeroberts @sgugger thank you for the review! All comments are addressed, I'll merge the branch once all tests are passing.<|||||>> Thanks for making these changes and improving the pipeline ⭐
> 
> I think the PR is good to go as is 👍 Would just like to see a bit more test coverage of the different segmentation tasks. Just one per task in the pipeline, possibly adapting existing ones to avoid making the test suite significantly slower. Have you visualised or counted the number of different pixels between the outputs on this branch and main as validation?

Thank you! Yes, I visualized the segments and they are the same.