Fix/fusion/reduce swap dims

[laggui]

Checklist

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

Related Issues/PRs

Closes #4815

Changes

Reduce broadcasted fuser requires contiguous inputs but having multi block local inputs might point to a reference that is SwapDims.

The current fix is to check for that condition, and refuse / close the fusion block.

Eventually we could improve fusion to support these cases.

Testing

All tests passing locally, except test_float_matmul_vecmat_transposed_fused (known issue, unrelated).

[bot-ember]

🏷️ Workflow test-gpu enabled 🟢
🕒 Auto-run scheduled on updates
ℹ A run has been scheduled... please wait for the results

[bot-ember]

❌ Workflow CI GPU:#4867 (run #389) failure!

[codecov]

Codecov Report

❌ Patch coverage is 59.37500% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 65.58%. Comparing base (87916c2) to head (ee50395).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...rates/burn-cubecl-fusion/src/engine/trace/fuser.rs	0.00%	7 Missing ⚠️
...fusion/src/optim/reduce_broadcasted/fuser/block.rs	0.00%	6 Missing ⚠️

❌ Your patch check has failed because the patch coverage (59.37%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.
❌ Your project check has failed because the head coverage (65.58%) is below the target coverage (80.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4867      +/-   ##
==========================================
- Coverage   65.71%   65.58%   -0.14%     
==========================================
  Files        1153     1154       +1     
  Lines      169104   169595     +491     
==========================================
+ Hits       111126   111222      +96     
- Misses      57978    58373     +395     

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