Initialize accumulator to bias for implicit GEMM to save an expensive float_add

[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

Loads the bias into the initial accumulator if present, to avoid an unnecessary extra binop.

Testing

All existing tests pass and no behaviour was changed

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 86 lines in your changes missing coverage. Please review.

Project coverage is 85.18%. Comparing base (eaf50e6) to head (87ad9fb).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
...s/burn-jit/src/kernel/conv/conv2d/implicit_gemm.rs	0.00%	86 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2383      +/-   ##
==========================================
- Coverage   85.23%   85.18%   -0.06%     
==========================================
  Files         785      785              
  Lines      103756   103823      +67     
==========================================
  Hits        88437    88437              
- Misses      15319    15386      +67     

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[nathanielsimard]

LGTM