wmk quantize crashes with FileExistsError when output .onnx.data sidecar already exists

[DingmaomaoBJTU]

Summary

Re-running `wmk quantize` with the same output path fails with `FileExistsError` if a `.onnx.data` sidecar file from a previous run already exists. The command has no overwrite or cleanup logic for external data files.

Repro

```bash

First run — succeeds

wmk quantize -m resnet_optimized.onnx --precision int8 -o resnet_quant_int8.onnx

Second run — crashes

wmk quantize -m resnet_optimized.onnx --precision int8 -o resnet_quant_int8.onnx
```

Error

```
Quantization failed:
Traceback (most recent call last):
File "src/winml/modelkit/quant/quantizer.py", line 166, in quantize_onnx
quantize(...)
...
File "onnx/external_data_helper.py", line 150, in convert_model_to_external_data
raise FileExistsError(f"External data file exists in {location}.")
FileExistsError: External data file exists in resnet_quant_int8.onnx.data.
```

Root Cause

In `src/winml/modelkit/quant/quantizer.py:163-164`, when `use_external_data=True` (the default), ORT's `quantize()` is called with `use_external_data_format=True`. ORT's `convert_model_to_external_data` raises `FileExistsError` if the sidecar `.data` file already exists — it does not overwrite.

`quantize_onnx` deletes/overwrites the `.onnx` file but does not check for or remove the accompanying `.onnx.data` sidecar before calling ORT quantization.

```python

quantizer.py:162-170

if use_external_data:
qdq_config.use_external_data_format = True
logger.info("Applying quantization...")
quantize(
model_input=str(model_path),
model_output=str(output_path), # .onnx written, but .onnx.data not cleaned up
quant_config=qdq_config,
)
```

Expected Behavior

`wmk quantize` should handle existing output paths gracefully — either:

• Delete the stale `.onnx.data` sidecar before writing (consistent with overwrite semantics for the `.onnx` file), or
• Raise a clear, user-friendly error message explaining that the sidecar file must be removed first

Acceptance Criteria

• Re-running `wmk quantize` with the same `-o` path succeeds without manual cleanup
• If `output_path.with_suffix(".onnx.data")` or any external data sidecar exists, it is deleted before quantization
• A test covers the re-run scenario (run quantize twice on same output, assert second run succeeds)

Related Files

• `src/winml/modelkit/quant/quantizer.py:86-90` — output path resolution
• `src/winml/modelkit/quant/quantizer.py:162-170` — where external data format is set and `quantize()` called
• `src/winml/modelkit/onnx/external_data.py` — `get_external_data_files()` can be used to enumerate sidecar files to clean up

[xieofxie]

Fixed by #320