[ONNX] STFT Support (#92087)
This PR addresses issue [#81075](https://github.com/pytorch/pytorch/issues/81075), making `torch.stft` compatible with ONNX Opset 17's STFT operator.
The conversion works for _most_ of `torch.stft` functionality:
- Batched or unbatched inputs
- Normalization
- Pre-computed windows
- Rectangular windows
- One-sided returns
- Window centering (implicitly supported)
What is currently _not_ supported is **complex types**, due to the lack of conversion functionality between PyTorch and ONNX (https://github.com/pytorch/pytorch/issues/86746).
Regardless, this is easy to bypass by setting `return_complex=False` when using `torch.stft`.
Note that there is already a draft PR to address this (https://github.com/pytorch/pytorch/pull/83944), but it is currently closed and it only partially addresses the conversion (i.e., most of `torch.stft` functionality is lacking, and unit tests are missing).
Pull Request resolved: https://github.com/pytorch/pytorch/pull/92087
Approved by: https://github.com/justinchuby
