Some Array rules

[willtebbutt]

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[codecov-commenter]

Codecov Report

Merging #491 (000b33f) into master (36508af) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #491      +/-   ##
==========================================
+ Coverage   98.04%   98.06%   +0.01%     
==========================================
  Files          22       22              
  Lines        2306     2321      +15     
==========================================
+ Hits         2261     2276      +15     
  Misses         45       45              

Impacted Files	Coverage Δ
src/rulesets/Base/array.jl	100.00% <100.00%> (ø)

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[willtebbutt]

@mcabbott @nickrobinson251 I've constrained the vect implementation a bit, because I couldn't figure out how to implement the most generic (and most useful) pullback. I've opened #492 to discuss further.

edit: (@nickrobinson251 I'll wait for you to approve before I merge, because I've modified thevect implementation a bit since you last looked at it)

[willtebbutt]

Seems like all comments have now been resolved -- @mcabbott I've requested a re-review because a lot has changed since your original approval.

[mcabbott]

I was wondering whether projects[n] gets handled well. @code_warntype seems happy... and my attempts to make things easier to unroll all make it slower:

julia> @btime rrule(Base.vect, 1,2,3)[2]($(rand(3)))
  28.684 ns (1 allocation: 112 bytes)
(NoTangent(), 0.7437540971290453, 0.6835525631785602, 0.29678387383966687)

julia> @btime rrule(Base.vect, 1+im,2+im,3+im)[2]($(rand(3)))
  235.140 ns (6 allocations: 416 bytes)
(NoTangent(), 0.9212083670665245 + 0.0im, 0.989459216141123 + 0.0im, 0.8454719840778347 + 0.0im)

julia> @btime rrule(Base.vect, 1, 2+3im, 4.0)[2]($(rand(3)))
  609.760 ns (6 allocations: 320 bytes)
(NoTangent(), 0.2914057312235363, 0.23309219863512798 + 0.0im, 0.08023319383991401)

julia> struct StaticGetter{i} end; @inline (::StaticGetter{i})(v) where {i} = v[i]; # from Zygote

julia> function rrule(
            ::typeof(Base.vect),
            X::Vararg{Union{Number,AbstractArray{<:Number}}, N},
        ) where {N}
            valN = Val(N)
            projects = map(ProjectTo, X)
            function vect_pullback(ȳ)
                X̄ = ntuple(n -> StaticGetter{n}()(projects)(ȳ[n]), valN)
                return (NoTangent(), X̄...)
            end
            return Base.vect(X...), vect_pullback
        end
rrule (generic function with 723 methods)

julia> @btime rrule(Base.vect, 1, 2+3im, 4.0)[2]($(rand(3)))
  1.442 μs (11 allocations: 448 bytes)
(NoTangent(), 0.7692288886268137, 0.39993377443044065 + 0.0im, 0.6341039234276757)

[willtebbutt]

Added one additional test -- will merge once CI passes.