[WIP] Migrate to cubecl IR refactor

[wingertge]

Pull Request Template

Checklist

• Confirmed that run-checks all script has been executed.
• Made sure the book is up to date with changes in this PR.

Related Issues/PRs

Depends on tracel-ai/cubecl#199

Changes

Migrates legacy kernels to the new Variable struct.

Testing

All tests pass on all backends (exempting one CUDA test that was already failing before).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.28%. Comparing base (d383478) to head (d24048c).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2418      +/-   ##
==========================================
- Coverage   85.30%   85.28%   -0.02%     
==========================================
  Files         792      792              
  Lines      104634   104511     -123     
==========================================
- Hits        89257    89133     -124     
- Misses      15377    15378       +1     

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