[ez] Expose FOLDED_FROM_KEY to onnxscript.optimizer public API (#2773)

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

```python
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.

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<details>

<summary>Original prompt</summary>

> 
> ----
> 
> *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)
> 
> <comments>
> </comments>
> 


</details>



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- Fixes #2772

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: justinchuby <11205048+justinchuby@users.noreply.github.com>

Author: Copilot

onnxscript/optimizer/__init__.py

@@ -8,6 +8,7 @@
     "basic_constant_propagation",
     "fold_constants_ir",
     "fold_constants",
+    "FOLDED_FROM_KEY",
     "inline",
     "optimize_ir",
     "optimize",
@@ -19,7 +20,7 @@
 
 import onnxscript.optimizer._constant_folding as constant_folding
 from onnxscript import ir
-from onnxscript.optimizer._constant_folding import basic_constant_propagation
+from onnxscript.optimizer._constant_folding import FOLDED_FROM_KEY, basic_constant_propagation
 from onnxscript.optimizer._constant_folding import fold_constants as fold_constants_ir
 from onnxscript.optimizer._optimizer import optimize_ir
 

onnxscript/optimizer/_optimizer_test.py

@@ -10,6 +10,12 @@
 
 
 class OptimizerTest(unittest.TestCase):
+    def test_folded_from_key_is_accessible(self):
+        """Test that FOLDED_FROM_KEY is accessible from the public API."""
+        self.assertTrue(hasattr(optimizer, "FOLDED_FROM_KEY"))
+        self.assertIsInstance(optimizer.FOLDED_FROM_KEY, str)
+        self.assertEqual(optimizer.FOLDED_FROM_KEY, "pkg.onnxscript.optimizer.folded_from")
+
     def _model_proto(self) -> onnx.ModelProto:
         return onnx.parser.parse_model(
             """