Integrate xlstm cleanly.

[kpoeppel]

What does this PR do?

This PR integrates xLSTM via the xlstm-library including certain optimizations (potentially use torch.compile and cuda graphs for speed up). This enables using the NX-AI/xLSTM-7b without a special fork of transformers.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline,
  Pull Request section?
• Was this discussed/approved via a Github issue or the forum? 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, and
  here are tips on formatting docstrings.
• Did you write any new necessary tests? Yes, I adapted the tests of the recurrent Mamba2 model.

Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

@ArthurZucker