Fix fusion reference selection when all inputs are reshaped views

[dcvz]

When fusing operations on multiple reshaped views of a cloned tensor, the fusion engine couldn't find a valid reference layout because none of the reshaped input shapes matched the block's expected broadcast shape. The fix creates a virtual reference using any available input tensor, overriding its shape with the expected iteration shape.

Fixes: #4327

[codecov]

Codecov Report

❌ Patch coverage is 0% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.95%. Comparing base (7bbf4df) to head (ff8322b).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...tes/burn-cubecl-fusion/src/engine/launch/output.rs	0.00%	15 Missing ⚠️

❌ Your patch check has failed because the patch coverage (0.00%) 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 (68.95%) 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    #4392      +/-   ##
==========================================
+ Coverage   68.90%   68.95%   +0.04%     
==========================================
  Files        1412     1413       +1     
  Lines      168459   168770     +311     
==========================================
+ Hits       116075   116370     +295     
- Misses      52384    52400      +16     

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