Use new FFT operators in stft (#47601)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/47601
Fixes https://github.com/pytorch/pytorch/issues/42175#issuecomment-719933913
Test Plan: Imported from OSS
Reviewed By: ngimel
Differential Revision: D25457217
Pulled By: mruberry
fbshipit-source-id: 455d216edd0b962eb7967ecb47cccc8d6865975b
