[ITensors] Fix broken broadcast operation on GPU (#1532)

NDTensors/test/test_dense.jl

@@ -83,6 +83,20 @@ NDTensors.dim(i::MyInd) = i.dim
         @test A[2, 2] == Aview[1, 1]
       end
 
+      ## Testing A .= α .* B .+ β .* A
+      C = copy(A)
+      @allowscalar fill!(B, zero(elt))
+      β = elt(2)
+      α = elt(1)
+      permutedims!!(A, B, (1, 2), (a, b) -> +(*(β, a), *(α, b)))
+      @allowscalar 2 .* C == A
+      randn!(B)
+      C = copy(A)
+      A = permutedims!!(A, B, (1, 2), (a, b) -> +(*(β, a), *(α, b)))
+      @allowscalar for i in 1:3, j in 1:4
+        @test A[i, j] == α * B[i, j] + β * C[i, j]
+      end
+
       ## add elt around 2.0 to preserve the eltype of A.
       @test data(A * elt(2.0)) == data(elt(2.0) * A)
 

src/broadcast.jl

@@ -395,6 +395,13 @@ end
 # C .= β .* C .+ α .* A .* B
 #
 
+struct axpby{Alpha,Beta} <: Function
+  alpha::Alpha
+  beta::Beta
+end
+
+(f::axpby)(y, x) = x * f.alpha + y * f.beta
+
 ## TODO this code doesn't actually get called
 function Base.copyto!(
   T::ITensor,
@@ -414,7 +421,9 @@ function Base.copyto!(
     A, C = C, A
   end
   if !isnothing(A) && !isnothing(C) && !isnothing(α) && !isnothing(β)
-    map!((r, t) -> β * r + α * t, T, T, A)
+    # The following fails to compile on some GPU backends.
+    # map!((r, t) -> β * r + α * t, T, T, A)
+    map!(axpby(α, β), T, T, A)
   else
     bc_bc_α = find_type(Broadcasted, bc_α.args)
     if isnothing(α)