Scalar indexing on GPU

[Moelf]

julia> using ArraysOfArrays, CUDA, Adapt

julia> cpu_ary=VectorOfVectors([rand(Float32, rand(1:5)) for _ = 1:3])
3-element VectorOfVectors{Float32, Vector{Float32}, Vector{Int64}, Vector{Tuple{}}}:
 Float32[0.7436946]
 Float32[0.6354585, 0.57725984]
 Float32[0.4983889, 0.11524224, 0.028724253, 0.77355224]

julia> gpu_ary = adapt(CuArray, cpu_ary);

julia> map(argmin, gpu_ary)
ERROR: Scalar indexing is disallowed.
Invocation of getindex resulted in scalar indexing of a GPU array.
This is typically caused by calling an iterating implementation of a method.
Such implementations *do not* execute on the GPU, but very slowly on the CPU,
and therefore should be avoided.

If you want to allow scalar iteration, use `allowscalar` or `@allowscalar`
to enable scalar iteration globally or for the operations in question.
Stacktrace:

[oschulz]

Yes, we need to add GPU computing support, via broadcast specialization. I don't think this should be too tricky, at least not for functions that have a scalar result.

StructArrays does it like this:

https://github.com/JuliaArrays/StructArrays.jl/blob/master/ext/StructArraysGPUArraysCoreExt.jl

We probably need something similar.