Tracking issue for plan chain rule support

[gaurav-arya]

Tracks progress towards supporting plan chain rules. This requires supporting adjoints of plans, and that in turn requires that we create a package for testing FFT plans to give downstream implementations a way of easily testing adjoint functionality.

• [x] Implement adjoints of plans (#67)
• [x] Create a TestUtils submodule and add adjoint plan testing (#78)
• [x] QOL improvements to adjoint styles (https://github.com/JuliaMath/AbstractFFTs.jl/pull/109)
• [ ] Implement adjoint traits in downstream packages (FFTW, CUDA, AMD, PencilFFTs) and test using TestUtils (https://github.com/JuliaMath/FFTW.jl/pull/249)
• [ ] drop Zygote FFT plan rules (https://github.com/FluxML/Zygote.jl/pull/1437)

Additionally, for full support of FFTW's additional transforms

• [] Adjoint support for R2R plans
• [] DCT chain rules (https://github.com/JuliaMath/FFTW.jl/pull/273)

And #112 needs resolution at some point.

[vpuri3]

Also https://github.com/JuliaMath/FFTW.jl/pull/273