Support for ISTFT

[alealv]

I would very much need support for ISTFT and I guess that given the fact that STFT is already supported this should be very easy to implement.

This is key for audio processing.

[junpeiz]

Thank you for the feature request. We put it in our feature request list and will add it when we have bandwidth. At the same time, if it's urgent on your side, feel free to try to add it by following this PR. Thanks!