fix Cuda gradients

fixes #195

This really needs some tests as the non-fast_scalar_indexing path was just completely broken.

src/gradients.jl

@@ -239,9 +239,8 @@ function finite_difference_gradient!(
     fx, c1, c2, c3 = cache.fx, cache.c1, cache.c2, cache.c3
     if fdtype != Val(:complex) && ArrayInterface.fast_scalar_indexing(c2)
         @. c2 = compute_epsilon(fdtype, one(eltype(x)), relstep, absstep, dir)
-        copyto!(c1, x)
     end
-    copyto!(c3, x)
+    copyto!(c1, x)
     if fdtype == Val(:forward)
         @inbounds for i ∈ eachindex(x)
             if ArrayInterface.fast_scalar_indexing(c2)
@@ -273,6 +272,7 @@ function finite_difference_gradient!(
             end
         end
     elseif fdtype == Val(:central)
+        copyto!(c3, x)
         @inbounds for i ∈ eachindex(x)
             if ArrayInterface.fast_scalar_indexing(c2)
                 epsilon = ArrayInterface.allowed_getindex(c2, i) * dir
@@ -296,9 +296,8 @@ function finite_difference_gradient!(
             ArrayInterface.allowed_setindex!(c3, x_old, i)
         end
     elseif fdtype == Val(:complex) && returntype <: Real
-        copyto!(c1, x)
-        epsilon_complex = eps(real(eltype(x)))
         # we use c1 here to avoid typing issues with x
+        epsilon_complex = eps(real(eltype(x)))
         @inbounds for i ∈ eachindex(x)
             c1_old = ArrayInterface.allowed_getindex(c1, i)
             ArrayInterface.allowed_setindex!(c1, c1_old + im * epsilon_complex, i)