[ez] Expose FOLDED_FROM_KEY to onnxscript.optimizer public API

[Copilot]

The FOLDED_FROM_KEY constant was previously only accessible from the private _constant_folding module, making it difficult for users to interact with constant folding metadata.

Changes

• Exported FOLDED_FROM_KEY in onnxscript/optimizer/__init__.py by adding it to __all__ and importing from _constant_folding
• Added test coverage to ensure the constant remains publicly accessible

Usage

from onnxscript.optimizer import FOLDED_FROM_KEY

# Use the key to access metadata about folded constants
folded_values = output.meta.get(FOLDED_FROM_KEY, set())

The constant value is "pkg.onnxscript.optimizer.folded_from", used to track which values were folded during constant folding optimization.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Expose FOLDED_FROM_KEY to public</issue_title>
<issue_description>Expose FOLDED_FROM_KEY to onnxscript.optimizer for public consumption.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Expose FOLDED_FROM_KEY to public #2772

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.46%. Comparing base (e45724c) to head (c17e046).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2773   +/-   ##
=======================================
  Coverage   70.45%   70.46%           
=======================================
  Files         228      228           
  Lines       27252    27256    +4     
  Branches     2759     2759           
=======================================
+ Hits        19201    19205    +4     
  Misses       7099     7099           
  Partials      952      952           

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