[5336829][AutoCast] Support subgraphs

[galagam]

Initial bug mentioned conditional operators, but the issue can be generalized to any subgraph in the ONNX. Support by recursively traversing subgraphs in PrecisionConverter.

What does this PR do?

Type of change: ? Bug fix

Overview:
Initial bug mentioned conditional operators, but the issue can be generalized to any subgraph in the ONNX.
Support by recursively traversing subgraphs in PrecisionConverter.

Testing

Add unit tests with conditional
tests/unit/onnx/autocast/test_precisionconverter.py::model_with_if_subgraph
tests/unit/onnx/autocast/test_precisionconverter.py::test_if_subgraph_initializer_conversion
tests/unit/onnx/autocast/test_precisionconverter.py::test_if_subgraph_mixed_precision_boundary

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?: Yes
• Did you add or update any necessary documentation?: No
• Did you update Changelog?: No

Additional Information

[galagam]

/bot run

[codecov]

Codecov Report

❌ Patch coverage is 87.32394% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.51%. Comparing base (53a2dde) to head (965b1ec).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
modelopt/onnx/autocast/precisionconverter.py	88.52%	7 Missing ⚠️
modelopt/onnx/autocast/utils.py	80.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #659      +/-   ##
==========================================
+ Coverage   74.50%   74.51%   +0.01%     
==========================================
  Files         183      183              
  Lines       18400    18439      +39     
==========================================
+ Hits        13709    13740      +31     
- Misses       4691     4699       +8     

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