Investigate using a different AD for tests

[ToucheSir]

Zygote compile times make what ordinarily should be a pretty fast test suite rather sluggish. Perhaps we could borrow some functionality from CRTU: https://github.com/JuliaDiff/ChainRulesTestUtils.jl/blob/v1.8.1/src/testers.jl#L224.

[mcabbott]

It would be nice to be quicker. But the tests aren't just about mathematical correctness, some are also checking that missing branches with nothing are handled correctly, etc. If JuliaDiff/Diffractor.jl#66 ever happens, these should ideally be supplemented by checks that ZeroTangent works equally well.

[ToucheSir]

Would FD.jl not generate NoTangent for those same branches? I've not tried it with nested structs.

[mcabbott]

It seems to make dense zeros:

julia> using FiniteDifferences

julia> grad(central_fdm(5, 1), x -> sum(x.a), (a=[1,2f0], b=([3,4f0], 5.0)))
((a = Float32[0.9999863, 1.0000029], b = (Float32[0.0, 0.0], 0.0)),)

I wondered if Tracker might work well, but it doesn't seem to like NamedTuples.

For comparison:

julia> Zygote.gradient(x -> sum(x.a), (a=[1,2f0], b=([3,4f0], 5.0)))
((a = Fill(1.0f0, 2), b = nothing),)

julia> Diffractor.gradient(x -> sum(x.a), (a=[1,2f0], b=([3,4f0], 5.0)))
(Tangent{NamedTuple{(:a, :b), Tuple{Vector{Float32}, Tuple{Vector{Float32}, Float64}}}}(a = InplaceableThunk(ChainRules.var"#1547#1550"{Float32, Colon}(1.0f0, Colon()), Thunk(ChainRules.var"#1548#1551"{Float32, Colon, Vector{Float32}, ProjectTo{AbstractArray, NamedTuple{(:element, :axes), Tuple{ProjectTo{Float32, NamedTuple{(), Tuple{}}}, Tuple{Base.OneTo{Int64}}}}}}(1.0f0, Colon(), Float32[1.0, 2.0], ProjectTo{AbstractArray}(element = ProjectTo{Float32}(), axes = (Base.OneTo(2),))))),),)

julia> ans[1].b
ZeroTangent()

[dfdx]

@mcabbott Since you linked the issue in Yota, I guess you want to test this example too. So let me save you a few minutes:

julia> grad(x -> sum(x.a), (a=[1,2f0], b=([3,4f0], 5.0)))[2][2:end]
(Tangent{NamedTuple{(:a, :b), Tuple{Vector{Float32}, Tuple{Vector{Float32}, Float64}}}}(a = Float32[1.0, 1.0],),)

julia> ans[1].b
ZeroTangent()

[cossio]

@mcabbott Funny that #105 explicitly says it won't close this. Maybe it was closed automatically because Github saw the "close #105" string.

Just checking if the there was a real intention to close this issue?