Add utility function for converting model protos to function proto

[chapman73]

This addresses the first part of: #2616

It tries to convert arbitrary model protos to function protos. Since function protos don't have a concept of initializers, they are made to be constants. This approach should address a large number of model protos (including 'models' from skl2onnx)

Happy for any particular feedback.

Note that this does not address the second part of #2616 , running the script with the operator embedded will not work.

[chapman73]

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[codecov]

Codecov Report

❌ Patch coverage is 86.95652% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.49%. Comparing base (8a94ad6) to head (96d7a1c).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
onnxscript/utils/model_proto_to_function_proto.py	86.95%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2655      +/-   ##
==========================================
+ Coverage   70.46%   70.49%   +0.03%     
==========================================
  Files         224      225       +1     
  Lines       26572    26595      +23     
  Branches     2637     2638       +1     
==========================================
+ Hits        18723    18749      +26     
+ Misses       6928     6924       -4     
- Partials      921      922       +1     

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[Copilot]

Pull Request Overview

This PR adds a utility function to convert ONNX model protos to function protos, addressing the first part of issue #2616. The key challenge is that function protos don't support initializers, so the implementation converts them to Constant nodes.

Key Changes:

• Implements convert_model_proto_to_function_proto() that transforms model initializers into Constant nodes
• Adds comprehensive test coverage for the conversion utility with multi-function model scenarios

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
onnxscript/utils/model_proto_to_function_proto.py	Core implementation of model-to-function proto conversion with initializer handling
tests/utils/model_proto_to_function_proto_test.py	Test suite validating conversion functionality with nested function calls
tests/utils/__init__.py	Package initialization file for the test utils module

[gramalingam]

Copy opset_import over?

[gramalingam]

Also: doc_string, metadata_props?

[gramalingam]

And value_info over from the graph ... we would also need to copy the value_info for graph inputs/outputs over