gather throw error on cpu and gpu array inputs

add tests

Author: yuehhua

src/gather.jl

@@ -63,3 +63,8 @@ function NNlib.gather!(dst::AnyCuArray, src::AnyCuArray, idx::AnyCuArray)
     kernel(args...; threads=threads, blocks=blocks)
     return dst
 end
+
+function NNlib.gather(src::AnyCuArray, idx::AbstractArray)
+    err_msg = "src and idx both must be on GPU, but received $(typeof(src)) and $(typeof(idx)), respectively."
+    throw(ArgumentError(err_msg))
+end

test/gather.jl

@@ -17,6 +17,7 @@
     gputest(src -> NNlib.gather(src, index), src, checkgrad=true)
     @test NNlib.gather!(CUDA.zeros(T, size(index)...), src, index) == output
     @test_throws ArgumentError NNlib.gather!(zeros(T, 3, 5), src, index)
+    @test_throws ArgumentError NNlib.gather(src, collect(index))
 
     ## 1d src, 2d index of tuples -> 2d output
     src = CT([3, 4, 5, 6, 7])