Merge #515

515: Fix Flux.flip by providing an adjoint for Base.reverse r=dhairyagandhi96 a=tanhevg

The main motivation behind this PR is to address various issues concerning `Flux.flip()` (used mainly for bRNNs), e.g.  FluxML/Flux.jl#962, FluxML/Flux.jl#990 and FluxML/model-zoo#179

Co-authored-by: Evgeny Tankhilevich <[email protected]>

Author: bors[bot]

src/lib/array.jl

@@ -63,6 +63,11 @@ end
   circshift(A, shifts), Δ -> (circshift(Δ, map(-, shifts)), nothing)
 end
 
+@adjoint function reverse(x::AbstractArray, args...; kwargs...)
+  _reverse(t) = reverse(t, args...; kwargs...)
+  _reverse(x), Δ->(_reverse(Δ), map(_->nothing, args)...)
+end
+
 @adjoint permutedims(xs) = permutedims(xs), Δ -> (permutedims(Δ),)
 
 @adjoint permutedims(xs::AbstractVector) = permutedims(xs), Δ -> (vec(permutedims(Δ)),)

test/gradcheck.jl

@@ -129,6 +129,11 @@ end
   @test gradtest(x -> meanpool(x, pdims), x)
 end
 
+@test gradtest(x -> reverse(x), rand(17))
+@test gradtest(x -> reverse(x, 8), rand(17))
+@test gradtest(x -> reverse(x, 8, 13), rand(17))
+@test gradtest(x -> reverse(x, dims=2), rand(17, 42))
+
 @test gradtest(x -> permutedims(x), rand(2))
 @test gradtest(x -> permutedims(x), rand(2,3))
 @test gradtest(x -> permutedims(x, [3,1,2]), rand(4,5,6))