Fix constant creation in fusion to cast at compile time, not runtime

[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.

Changes

Changes the fusion read kernel to cast literals at compile time, not runtime.

Testing

All tests pass, and issues encountered in https://github.com/ArthurBrussee/brush/tree/spirv have been fixed.

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 83.63%. Comparing base (b9653f5) to head (aea078e).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/burn-jit/src/fusion/on_write/io.rs	75.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2782      +/-   ##
==========================================
- Coverage   83.64%   83.63%   -0.01%     
==========================================
  Files         826      826              
  Lines      108746   108752       +6     
==========================================
+ Hits        90956    90960       +4     
- Misses      17790    17792       +2     

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[laggui]

Thanks for the quick fix!

Just a comment regarding the cubecl wgpu re-exports.