rrule for stack (#681)

* rrule for stack

* bump version

* extend rrule to muldim containers

* hope you don't mind me committing these

* import stack in tests

* cleanup

* Apply3 suggestions

Co-authored-by: Michael Abbott <32575566+mcabbott@users.noreply.github.com>

Project.toml

@@ -1,6 +1,6 @@
 name = "ChainRules"
 uuid = "082447d4-558c-5d27-93f4-14fc19e9eca2"
-version = "1.44.7"
+version = "1.45.0"
 
 [deps]
 Adapt = "79e6a3ab-5dfb-504d-930d-738a2a938a0e"
@@ -20,7 +20,7 @@ StructArrays = "09ab397b-f2b6-538f-b94a-2f83cf4a842a"
 Adapt = "3.4.0"
 ChainRulesCore = "1.15.3"
 ChainRulesTestUtils = "1.5"
-Compat = "3.42.0, 4"
+Compat = "3.46, 4.2"
 FiniteDifferences = "0.12.20"
 GPUArraysCore = "0.1.0"
 IrrationalConstants = "0.1.1"

src/ChainRules.jl

@@ -22,6 +22,8 @@ import ChainRulesCore: rrule, frule
 # Experimental:
 using ChainRulesCore: derivatives_given_output
 
+using Compat: stack
+
 # numbers that we know commute under multiplication
 const CommutativeMulNumber = Union{Real,Complex}
 

src/rulesets/Base/array.jl

@@ -610,3 +610,30 @@ function _extrema_dims(x, dims)
     end
     return y, extrema_pullback_dims
 end
+
+#####
+##### `stack`
+#####
+
+function frule((_, ẋ), ::typeof(stack), x; dims::Union{Integer, Colon} = :)
+    return stack(x; dims), stack(ẋ; dims)
+end
+
+# Other iterable X also allowed, maybe this should be wider?
+function rrule(::typeof(stack), X::AbstractArray; dims::Union{Integer, Colon} = :)
+    Y = stack(X; dims)
+    sdims = if dims isa Colon
+        N = ndims(Y) - ndims(X)
+        X isa AbstractVector ? ndims(Y) : ntuple(i -> i + N, ndims(X))
+    else
+        dims
+    end
+    project = ProjectTo(X)
+    function stack_pullback(Δ)
+        dY = unthunk(Δ)
+        dY isa AbstractZero && return (NoTangent(), dY)
+        dX = collect(eachslice(dY; dims = sdims))
+        return (NoTangent(), project(reshape(dX, project.axes)))
+    end
+    return Y, stack_pullback
+end

test/rulesets/Base/array.jl

@@ -416,3 +416,33 @@ end
     B = hcat(A[:,:,1], A[:,:,1])
     @test extrema(B, dims=2) == rrule(extrema, B, dims=2)[1]
 end
+
+@testset "stack" begin
+    # vector container
+    xs = [rand(3, 4), rand(3, 4)]
+    test_frule(stack, xs)
+    test_frule(stack, xs; fkwargs=(dims=1,))
+
+    test_rrule(stack, xs, check_inferred=false)
+    test_rrule(stack, xs, fkwargs=(dims=1,), check_inferred=false)
+    test_rrule(stack, xs, fkwargs=(dims=2,), check_inferred=false)
+    test_rrule(stack, xs, fkwargs=(dims=3,), check_inferred=false)
+
+    # multidimensional container
+    ms = [rand(2,3) for _ in 1:4, _ in 1:5];
+
+    if VERSION > v"1.9-"  # this needs new eachslice, not yet in Compat
+        test_rrule(stack, ms, check_inferred=false)
+    end
+    test_rrule(stack, ms, fkwargs=(dims=1,), check_inferred=false)
+    test_rrule(stack, ms, fkwargs=(dims=3,), check_inferred=false)
+
+    # non-array inner objects
+    ts = [Tuple(rand(3)) for _ in 1:4, _ in 1:2];
+
+    if VERSION > v"1.9-"  
+        test_rrule(stack, ts, check_inferred=false)
+    end
+    test_rrule(stack, ts, fkwargs=(dims=1,), check_inferred=false)
+    test_rrule(stack, ts, fkwargs=(dims=2,), check_inferred=false)
+end

test/runtests.jl

@@ -5,6 +5,7 @@ using Test, ChainRulesCore, ChainRulesTestUtils
 using Adapt
 using Base.Broadcast: broadcastable
 using ChainRules
+using ChainRules: stack
 using ChainRulesCore
 using ChainRulesTestUtils
 using ChainRulesTestUtils: rand_tangent, _fdm