`onehotbatch(::CuArray, ...)` moves data to host

The lack of https://github.com/FluxML/Flux.jl/pull/1959 causes the following error, currently blocking https://github.com/FluxML/Flux.jl/pull/2025 :

```julia
julia> using CUDA, OneHotArrays, NNlibCUDA

julia> CUDA.allowscalar(false)

julia> x = [1, 3, 2];

julia> y = onehotbatch(x, (0,1,2,3))
4×3 OneHotMatrix(::Vector{UInt32}) with eltype Bool:
⋅ ⋅ ⋅
1 ⋅ ⋅
⋅ ⋅ 1
⋅ 1 ⋅

julia> y2 = onehotbatch(x |> cu, (0,1,2,3))
ERROR: Scalar indexing is disallowed.
```
Edit: after #27, `onehotbatch(x |> cu, 0:3)` works, but other ways to specify the labels do not.