Fixing ONNX error

Updating path variable to a string to avoid the following error
```Error during ONNX export: Function torch.onnx.utils.export() parameter f="PosixPath('htdemucs.onnx')" violates type hint typing.Union[str, _io.BytesIO], as <protocol "pathlib.PosixPath"> "PosixPath('htdemucs.onnx')" not str or <class "_io.BytesIO">.```

Author: writer-in-fancy-pants

scripts/convert-pth-to-onnx.py

@@ -48,7 +48,7 @@
         torch.onnx.export(
             core_model,
             dummy_input,
-            onnx_file_path,
+            str(onnx_file_path),
             export_params=True,
             opset_version=17,
             do_constant_folding=True,