Add PocketFFT support (#60976)
Summary:
Needed on platforms, that do not have MKL, such as aarch64 and M1
- Add `AT_POCKETFFT_ENABLED()` to Config.h.in
- Introduce torch._C.has_spectral that is true if PyTorch was compiled with either MKL or PocketFFT
- Modify spectral test to use skipCPUIfNoFFT instead of skipCPUIfNoMKL
Share implementation of `_out` functions as well as fft_fill_with_conjugate_symmetry_stub between MKL and PocketFFT implementations
Fixes https://github.com/pytorch/pytorch/issues/41592
Pull Request resolved: https://github.com/pytorch/pytorch/pull/60976
Reviewed By: walterddr, driazati, janeyx99, samestep
Differential Revision: D29466530
Pulled By: malfet
fbshipit-source-id: ac5edb3d40e7c413267825f92a5e8bc4bb249caf
