Fix ONNX linear bug by not updating shape for linear if not present

[antimora]

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

#1560

Changes

Input shapes are not always present, so we skip updating the output if not known.

Testing

Tested on the user provided ONNX file in #1560

[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 86.51%. Comparing base (b387829) to head (1388afa).
Report is 2 commits behind head on main.

Files	Patch %	Lines
crates/burn-import/src/onnx/dim_inference.rs	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1700      +/-   ##
==========================================
- Coverage   86.51%   86.51%   -0.01%     
==========================================
  Files         696      696              
  Lines       81499    81498       -1     
==========================================
- Hits        70508    70506       -2     
- Misses      10991    10992       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.