[5643020] AutoCast use onnxscript for opset conversion

[galagam]

What does this PR do?

Type of change: Bug fix

Overview:
Opset conversion for large models (with external data) is not handled well in onnx.
PR onnx/onnx#6847 allows opset conversion if loading the model without weights.
While this is a possible WAR, it requires a major change in AutoCast, reloading the model as part of GraphSanitizer.
Onnxscript handles opset conversion for large models better, and is already a dependency of the ModelOpt ONNX package.

Usage

# Add a code snippet demonstrating how to use this

Testing

Before your PR is "Ready for review"

• Make sure you read and follow Contributor guidelines and your commits are signed.
• Is this change backward compatible?: Yes
• Did you write any new necessary tests?: Not applicable
• Did you add or update any necessary documentation?: No
• Did you update Changelog?: No

Additional Information

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.38%. Comparing base (7d0f7a9) to head (d23f1a4).
⚠️ Report is 12 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #542   +/-   ##
=======================================
  Coverage   74.37%   74.38%           
=======================================
  Files         182      182           
  Lines       18219    18225    +6     
=======================================
+ Hits        13550    13556    +6     
  Misses       4669     4669           

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