test restructure on the GPU

Requires https://github.com/FluxML/Zygote.jl/pull/474

test/cuda/cuda.jl

@@ -58,6 +58,13 @@ end
   @test y[3,:] isa CuArray
 end
 
+@testset "restructure gpu" begin
+  dudt = Dense(1,1) |> gpu
+  p,re = Flux.destructure(dudt)
+  foo(x) = sum(re(p)(x))
+  @test gradient(foo, cu(rand(1)))[1] isa CuArray
+end
+
 if CuArrays.has_cudnn()
   @info "Testing Flux/CUDNN"
   include("cudnn.jl")