fix: make gpu(x) return unmodified x when GPU backends aren't loaded

src/functor.jl

@@ -341,6 +341,7 @@ function gpu(::FluxCUDAAdaptor, x)
         `CUDA.jl` must be loaded to access it.
         Add `using CUDA` or `import CUDA` to your code.
         """ maxlog=1
+        return x
     end
 end
 
@@ -361,6 +362,7 @@ function gpu(::FluxAMDAdaptor, x)
         `AMDGPU.jl` must be loaded to access it.
         Add `using AMDGPU` or `import AMDGPU` to your code.
         """ maxlog=1
+        return x
     end
 end
 
@@ -380,6 +382,7 @@ function gpu(::FluxMetalAdaptor, x)
         The Metal functionality is being called but
         `Metal.jl` must be loaded to access it.
         """ maxlog=1
+        return x
     end
 end
 

test/functors.jl

@@ -0,0 +1,10 @@
+x = rand(Float32, 10, 10)
+if Flux.CUDA_LOADED[]
+    @test x !== gpu(x)
+elseif Flux.AMD_LOADED[]
+    @test x !== gpu(x)
+elseif Flux.METAL_LOADED[]
+    @test x !== gpu(x)
+else
+    @test x === gpu(x)
+end

test/runtests.jl

@@ -56,6 +56,9 @@ Random.seed!(0)
     include("outputsize.jl")
   end
 
+  @testset "functors" begin
+    include("functors.jl")
+  end
 
   if get(ENV, "FLUX_TEST_CUDA", "false") == "true"
     using CUDA