torch.fft: Multi-dimensional transforms (#44550)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/44550
Part of the `torch.fft` work (gh-42175).
This adds n-dimensional transforms: `fftn`, `ifftn`, `rfftn` and `irfftn`.
This is aiming for correctness first, with the implementation on top of the existing `_fft_with_size` restrictions. I plan to follow up later with a more efficient rewrite that makes `_fft_with_size` work with arbitrary numbers of dimensions.
Test Plan: Imported from OSS
Reviewed By: ngimel
Differential Revision: D23846032
Pulled By: mruberry
fbshipit-source-id: e6950aa8be438ec5cb95fb10bd7b8bc9ffb7d824
