fix: Add einops as a core dependency

[Hashbrownsss]

What does this PR do?

This PR addresses the ModuleNotFoundError for the einops library by adding it as a core dependency.

It fixes issue #39811, where certain parts of the transformers library, specifically in the flash attention code, were trying to import einops without it being a declared dependency. This bug prevented users from successfully running models that rely on this library.

By adding einops to setup.py, this PR ensures that the library is automatically installed for all users, making the transformers package more robust and reliable.

Fixes #39811

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. This PR fixes issue Missing einops dependency causing ModuleNotFoundError #39811.
• Did you make sure to update the documentation with your changes? Here are the
        documentation guidelines, and
        here are tips on formatting docstrings.

  This change is a dependency fix and does not require a documentation update.

• Did you write any new necessary tests?

  This change is a dependency fix and does not require a new test. The existing test suite was used to confirm that the einops dependency is now correctly installed.

Who can review?

@ArthurZucker
@ivarflakstad
@iforgetmyname

[ArthurZucker]

Thanks! Tho I think this should be added to the npu extra instead!

[Hashbrownsss]

Hey! Thanks for the quick feedback. Although i couldnt find a dedicated npu extra. ive updated the PR to add einops to dev-torch instead, as it seems like the most apt place for the dependency I hope that works!

[vasqu]

It seems like the einops dependency is only due to the un-/padding functions. It would be best to just rewrite them with normal torch and/or use the fa3 equivalents in the fa modeling utils file - no need for the einops dependency.

[vasqu]

#40002 will probably remove the einops dependency but will keep you updated if not

[Hashbrownsss]

That sounds good! I'll close this PR and keep an eye on #40002