Implement shape merging in identity elimination pass

[justinchuby]

This PR implements shape merging functionality in the identity elimination pass to preserve shape information when eliminating redundant Identity nodes.

Following microsoft/onnxscript#2588.

[Copilot]

Pull Request Overview

This PR implements shape merging functionality in the identity elimination pass to preserve shape information when eliminating redundant Identity nodes. The change builds on previous work to handle shape metadata during optimization.

• Adds a _merge_shapes helper function to intelligently combine shape information from input and output values
• Updates the identity elimination logic to preserve the most complete shape and type information available

[codecov]

Codecov Report

❌ Patch coverage is 23.80952% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.58%. Comparing base (49a361c) to head (fbc9fbf).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/onnx_ir/passes/common/identity_elimination.py	23.80%	11 Missing and 5 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #206      +/-   ##
==========================================
- Coverage   76.80%   76.58%   -0.23%     
==========================================
  Files          40       40              
  Lines        4915     4936      +21     
  Branches      978      986       +8     
==========================================
+ Hits         3775     3780       +5     
- Misses        856      867      +11     
- Partials      284      289       +5     

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