Fix protobuf serialization failure in ort_fusions ort_run for large models

[Copilot]

TestFuseXformers.test_fuse_xformers intermittently failed with google.protobuf.message.EncodeError: Failed to serialize proto (observed on the onnx_weekly Windows CI).

The ort_run test helper serialized the whole model in memory via ModelProto.SerializeToString(). The SmolLM test model is ~671MB — dominated by the 49152×2048 head_weight (~402MB) — close enough to protobuf's 2GB single-message serialization ceiling that in-memory serialization can fail depending on platform/library version.

Changes

• onnxscript/rewriter/ort_fusions/_test_utils.py: ort_run now writes the model to a temp file with external data via ir.save and loads it into InferenceSession by path, bypassing the in-memory protobuf size limit. ir.save leaves the in-memory model unchanged, so repeated ort_run calls on the same model still work.

with tempfile.TemporaryDirectory() as temp_dir:
    model_path = os.path.join(temp_dir, f"{model_name}.onnx")
    ir.save(model, model_path, external_data=f"{model_name}.onnx.data")
    session = onnxruntime.InferenceSession(model_path, options, providers=providers)
    return session.run(None, inputs)

This helper is shared across the ort_fusions test suite, so the fix applies to all fusion tests that round-trip models through ORT.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.65%. Comparing base (69a6938) to head (c0b2527).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2955   +/-   ##
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  Coverage   72.65%   72.65%           
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  Files         259      259           
  Lines       31766    31770    +4     
  Branches     3007     3007           
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+ Hits        23078    23082    +4     
  Misses       7670     7670           
  Partials     1018     1018           

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