Remove istft python functional wrapper (#71993)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/71993
I've kept the symbol `torch.functional.istft` because it looks like public API,
but it could just as easily be moved to `_torch_docs.py`.
Moving this into its own PR until TorchScript starts recognizing `input`
as a keyword argument.
Test Plan: Imported from OSS
Reviewed By: mrshenli
Differential Revision: D34461399
Pulled By: anjali411
fbshipit-source-id: 3275fb74bef2fa0e030e61f7ee188daf8b5b2acf
(cherry picked from commit 5b4b083de894eba9ab16cea53b77746bcfd0fe32)
