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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[Copilot]

Pull request overview

This PR exposes the internal constant-folding registration decorator as a public API, enabling users to register custom constant folders via onnxscript.optimizer.register_constant_folder instead of importing from a private module.

Changes:

• Re-exported _constant_folding.register as the public register_constant_folder API and added it to onnxscript.optimizer.__all__.
• Added documentation to PartialEvaluatorRegistry.register describing the expected callback signature and providing an example.
• Added a unit test that registers a custom folder under a test-only domain and verifies it is invoked by fold_constants.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
onnxscript/optimizer/__init__.py	Re-exports the registration decorator as register_constant_folder and adds it to the public API surface.
onnxscript/optimizer/_constant_folding.py	Adds docstring documentation for the registration decorator/signature and a usage example.
onnxscript/optimizer/_constant_folding_test.py	Adds a unit test covering public registration and invocation during constant folding.

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated no new comments.

Comments suppressed due to low confidence (2)

onnxscript/optimizer/_constant_folding_test.py:884

• This test is meant to demonstrate the public optimizer.register_constant_folder API, but the callback signature type-annotates state with the private _constant_folding.OptimizerState. That undermines the goal of avoiding private-member references in user-facing examples.

        def fold_custom_add(node: ir.Node, op, state: _constant_folding.OptimizerState):

onnxscript/optimizer/_constant_folding.py:284

• The new public API’s docstring refers to internal types (OptimizerContext/OptimizerState/ReturnValue) and uses an example that isn’t self-contained (it assumes register_constant_folder is already in scope). Since this docstring is what users will see via onnxscript.optimizer.register_constant_folder, it should avoid requiring private imports and clarify how the op builder is intended to be used.

        """Register a custom constant folding rule for an operator.

        The decorated function must have the signature:
        `(node: ir.Node, op: OptimizerContext, state: OptimizerState) -> ReturnValue`

[RohanMali2003]

Hi @justinchuby , gentle ping on this PR when you have a chance!

Also, I noticed the Copilot bot just flagged that the callback signature in the test and docstring still references the internal OptimizerContext and OptimizerState types. As I noted in the PR description, I held off on making those public to keep the scope tight, but I am more than happy to expose them as well if you'd prefer! Just let me know what direction you want to take. thanks!

[justinchuby]

Thanks! This is a welcomed change but I may need to take some time before I can review because my plate is full with other work.

[RohanMali2003]

No problem at all, and thanks for the heads-up, boss! Take your time, and I'll address any feedback whenever you get the chance to review. No rush! :)

[codecov]

Codecov Report

❌ Patch coverage is 90.47619% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.58%. Comparing base (9ef0aac) to head (2fa1f97).

Files with missing lines	Patch %	Lines
onnxscript/optimizer/_constant_folding_test.py	89.47%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2967      +/-   ##
==========================================
- Coverage   72.63%   72.58%   -0.05%     
==========================================
  Files         265      265              
  Lines       32204    32224      +20     
  Branches     3041     3043       +2     
==========================================
  Hits        23391    23391              
- Misses       7779     7794      +15     
- Partials     1034     1039       +5     

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