C++ API and docs for hfftn (#66127)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/66127
cc mruberry peterbell10
Test Plan: Imported from OSS
Reviewed By: dagitses
Differential Revision: D31450216
Pulled By: mruberry
fbshipit-source-id: 2878aee294aa7d74482b66d536258bac0541408d
