Fix image segmentation pipeline errors, resolve backward compatibility issues
# What does this PR do?
- Replaces hard-coded mask score binarization threshold used in `post_process_panoptic_segmentation` and `post_process_instance_segmentation` method of DETR and MaskFormer feature extractors with the `mask_threshold` argument
- Adds copied from statements to MaskFormer post processing helper functions
- Reintroduces the mask_threshold argument to the `ImageSegmentationPipeline` (resolves backward compatibility issues)
- Renames `ImageSegmentationPipeline` task argument to subtask to avoid conflicts, defaults to None
- If `ImageSegmentationPipeline` subtask is unset, tries performing panoptic segmentation first, followed by instance and semantic segmentation
- Fixes broken `ImageSegmentationPipeline` tests

Reintroducing the `mask_threshold` argument and setting it to 0 fixes the test errors and ensures at least one mask is returned for any input and 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?
- [X ] 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._