Add type annotation to stft (#21302)
Summary:
We want to be able to call stft from a torchscript which requires that stft have a type annotation
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21302
Differential Revision: D15607973
Pulled By: cpuhrsch
fbshipit-source-id: c4a5c09cdaafe7e81cf487a3ad216d1b03464a21
