Fixed gradient checkpoint bug for TimeSeriesTransformer
# What does this PR do?

Moved gradient checkpointing clause to above the decoder layer implementation. This should fix the bug this issue addresses.

Fixes #21737 


## 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.
[GitHub Issue](https://github.com/huggingface/transformers/pull/21733)
- [ ] 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?


## Who can review?

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@younesbelkada 


_The documentation is not available anymore as the PR was closed or merged._<|||||>Strange, I will see if I can install Python for my environment then. Thank you for all your help, and thanks for running make for me! 