Fix 29807, sinusoidal positional encodings overwritten by post_init()

[hovnatan]

What does this PR do?

Fixes # 29807, sinusoidal positional encodings overwritten by post_init(). First time contributing, please let me know of any issues, comments.

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, Pull Request section?
• [x] Was this discussed/approved via a Github issue or the forum? Please add a link to it if that's the case. https://github.com/huggingface/transformers/issues/29807
• [ ] Did you make sure to update the documentation with your changes? Here are the documentation guidelines, and here are tips on formatting docstrings.
• [x] Did you write any new necessary tests?

Who can review?

@ArthurZucker, @younesbelkada, @amyeroberts