Fix BeitFeatureExtractor postprocessing
# What does this PR do?
- Fixes a `BeitFeatureExtractor.post_process_semantic_segmentation()` assertion error when no `target_sizes` argument is provided
- Ensures post_process_semantic_segmentation returns a list of int64 PyTorch tensors
- Adds a test to ensure correct post-processing

## 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._<|||||>Hey @alaradirik, please also ping a core maintainer for review before merging PRs.