Change Attr __str__ to use repr()

[justinchuby]

When the attribute is string, for example, we want to display "attr" instead of attr.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 76.71%. Comparing base (745f68a) to head (fbf0ed6).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/onnx_ir/_core.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #190   +/-   ##
=======================================
  Coverage   76.71%   76.71%           
=======================================
  Files          40       40           
  Lines        4896     4896           
  Branches      974      974           
=======================================
  Hits         3756     3756           
  Misses        855      855           
  Partials      285      285           

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