Fix sentinel token IDs in data collator for Flax T5 pretraining script
# What does this PR do?

Modifies the sentinel token IDs used in the data collator for the Flax T5 pretraining script so that they go in decreasing order starting at `len(tokenizer) - 1`, which matches the [original T5 code](https://github.com/google-research/text-to-text-transfer-transformer/blob/main/t5/data/preprocessors.py#L2895).

Fixes #14282 


## 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/master/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.
- [ ] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/master/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?
Hey @rahuln! I'm pinging @patrickvonplaten for review as you have worked with him until now, but please note that he's off until next week so he'll review your PR when he's back! Thanks for your understanding.<|||||>Great! Thanks a lot for digging into this issue and fixing it