Remove unused operator at::_fft_with_size (#48905)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/48905
Test Plan: Imported from OSS
Reviewed By: ngimel
Differential Revision: D25480385
Pulled By: mruberry
fbshipit-source-id: 192d04a1b7e33b4e408cda8a82679c3ae3490a7d
