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handle permutedims rrule with ZeroTangent #683

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Dec 3, 2022
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handle permutedims rrule with ZeroTangent #683

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3 changes: 3 additions & 0 deletions src/rulesets/Base/array.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -143,18 +143,21 @@ end
function rrule(::typeof(permutedims), x::AbstractVector)
project = ProjectTo(x)
permutedims_pullback_1(dy) = (NoTangent(), project(permutedims(unthunk(dy))))
permutedims_pullback_1(::ZeroTangent) = (NoTangent(), ZeroTangent())
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IDK why there are several Zeros, but perhaps if doing this it may as well accept all of them:

Suggested change
permutedims_pullback_1(::ZeroTangent) = (NoTangent(), ZeroTangent())
permutedims_pullback_1(z::AbstractZero) = (NoTangent(), z)

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IDK either, but I follow how CR.jl previously do.

ChainRules.jl/src/rulesets/LinearAlgebra/norm.jl

Line 59 in a9a84ba

norm_pullback_p(::ZeroTangent) = (NoTangent(), ZeroTangent(), ZeroTangent())

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Instances of the wider pattern, which I didn't write, include:

ChainRules.jl/src/rulesets/SparseArrays/sparsematrix.jl

Line 33 in b1daa7a

V̄ isa AbstractZero && return (NoTangent(), V̄)

ChainRules.jl/src/rulesets/SparseArrays/sparsematrix.jl

Line 33 in b1daa7a

V̄ isa AbstractZero && return (NoTangent(), V̄)

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Oh, then should I update this PR, or just leave as it is?

return permutedims(x), permutedims_pullback_1
end

function rrule(::typeof(permutedims), x::AbstractArray, perm)
pr = ProjectTo(x) # projection restores e.g. transpose([1,2,3])
permutedims_back_2(dy) = (NoTangent(), pr(permutedims(unthunk(dy), invperm(perm))), NoTangent())
permutedims_back_2(::ZeroTangent) = (NoTangent(), ZeroTangent(), NoTangent())
return permutedims(x, perm), permutedims_back_2
end

function rrule(::Type{<:PermutedDimsArray}, x::AbstractArray, perm)
pr = ProjectTo(x)
permutedims_back_3(dy) = (NoTangent(), pr(permutedims(unthunk(dy), invperm(perm))), NoTangent())
permutedims_back_3(::ZeroTangent) = (NoTangent(), ZeroTangent(), NoTangent())
return PermutedDimsArray(x, perm), permutedims_back_3
end

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