Update feature selection in to_tf_dataset
# What does this PR do?

Updates feature selection to ensure returned dataset structure is consistent after merging of datasets PR: https://github.com/huggingface/datasets/pull/5602. The PR makes it possible to return a TF dataset with a dict structure even if only a single feature is selected. 

Compatibility with this version of datasets was run with [this commit](https://github.com/huggingface/transformers/pull/21935/commits/b64204bc1093edd7e3666ad76354fa09405cf4ec) and had a [successful run](https://github.com/huggingface/transformers/actions/runs/4555221658/jobs/8034018899).

Note: In all the cases here, the examples were tested with and without these updates. The models would successfully train with both the new and old dataset structures. 

## Before submitting
- [x] 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.
- [ ] 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._<|||||>cc @MKhalusova Reference on updated API for selecting features from TF datasets<|||||>Thanks for updating the docs too! Looks neat :)<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md) are likely to be ignored.