Expose the register as public register_constant_folder API (fix for Issue #2507)

[RohanMali2003]

Summary

Exposes the internal constant-folding registration decorator as a public API: onnxscript.optimizer.register_constant_folder.

Previously, registering a custom constant-folding rule required importing the protected register function from onnxscript.optimizer._constant_folding directly, which linters flag as a private-member import. This re-exports it under a public, more descriptive name, following the same pattern used in #2773 for FOLDED_FROM_KEY.

Closes #2507

Changes

• onnxscript/optimizer/__init__.py: re-export _constant_folding.register as register_constant_folder, added to __all__.
• onnxscript/optimizer/_constant_folding.py: added a docstring to PartialEvaluatorRegistry.register documenting the callback signature, parameters, and a usage example.
• onnxscript/optimizer/_constant_folding_test.py: added test_register_constant_folder, which registers a folder for a mock op under a dedicated test domain (test.custom), verifies fold_constants invokes it, and cleans up the registration in a finally block.

Scope notes

• Only the register decorator is exposed; other PartialEvaluatorRegistry internals (e.g. lookup_evaluators) remain private.
• The callback signature references OptimizerContext/OptimizerState, which are currently internal types. I've documented the expected signature informally in the docstring rather than exposing those types — happy to expose them too if preferred.

Testing

• pytest onnxscript/optimizer/_constant_folding_test.py -v — all passing
• pytest --doctest-modules onnxscript/optimizer — all passing
• pytest tests/optimizer/test_models.py -v — all passing (broader sanity check)
• lintrunner — clean

Sidenote:

I developed and documented this with AI-assisted tooling; all changes were reviewed, and verified by running the full test suite, doctests, and lintrunner locally before submission.

[RohanMali2003]

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