Fix unicode issue

[gramalingam]

This copilot-suggested fix enabled converting a recent onnx model to onnxscript.

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.09%. Comparing base (70abdd9) to head (c494888).
⚠️ Report is 12 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
onnxscript/backend/onnx_export.py	66.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2747      +/-   ##
==========================================
- Coverage   70.10%   70.09%   -0.01%     
==========================================
  Files         228      228              
  Lines       27394    27385       -9     
  Branches     2781     2783       +2     
==========================================
- Hits        19204    19195       -9     
+ Misses       7234     7231       -3     
- Partials      956      959       +3     

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