Fixes fft function calls for C++ API (#43749)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/43732.
Requires importing the fft namespace in the C++ API, just like the Python API does, to avoid clobbering torch::fft the function.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/43749
Reviewed By: glaringlee
Differential Revision: D23391544
Pulled By: mruberry
fbshipit-source-id: d477d0b6d9a689d5c154ad6c31213a7d96fdf271
