Support for iSTFT

## 🌱 Describe your Feature Request
I am trying to convert a Music Source Separation (MSS) Pytorch models using CoreMLTools using this guide https://apple.github.io/coremltools/docs-guides/source/convert-pytorch-workflow.html to later integrate with my inference implementation in Swift. The model uses both `torch.stft` and `torch.istft` in its functionality. The tracing stops in error of absence of iSTFT conversion functionality.

## How can this feature be used?
Inverse STFT is essential functionality in MSS and other audio applications.

## Describe alternatives you've considered
None yet, to work around this any pointers are appreciated.

Start with the PyTorch conversion workflow linked in the issue and trace where torch.stft conversion is handled, then compare that path with the torch.istft failure. Review the converter's existing operator support and determine the validation needed for inverse STFT. Done means the MSS model converts successfully through the documented workflow.

Written by the indexing model from the issue text.