Fixing ONNX error

[writer-in-fancy-pants]

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">.

There's also an issue with running the conversion with newer onnx versions >1.19

Error during ONNX export: STFT does not currently support complex types  [Caused by the value '636 defined in (%636 : Float(*, *, strides=[351232, 1], requires_grad=0, device=cpu) = onnx::Reshape[allowzero=0](%626, %635), scope: demucs.htdemucs.HTDemucs:: # <env>/lib/python3.10/site-packages/torch/functional.py:659:0
)' (type 'Tensor') in the TorchScript graph. The containing node has kind 'onnx::Reshape'.] 

[acolombier]

LGTM, thanks!