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 4 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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[justinchuby]

I have seen cases where the string attributes iare used to store binary data (e.g. in the custom Tokenizer op, even though this is invalid onnx). Is this trying to address those cases? If so, I would simply preserve the b-string to avoid accidental modifications.

Related: onnx/ir-py#184

[gramalingam]

I don't know. All I know is that this enabled me to convert a recent model (Word Fluency) into onnxscript so that I could see the control-flow structure of the model in a compact readable format. I don't mind alternatives if they allow me to do this.

[justinchuby]

Right - that's the same model I looked at. Making a suggestion

[justinchuby]

Can you try the current change?