add bounds checking in `gather`

Currently `NNlibCUDA.gather!` would not check bounds 
```julia
using NNlib, CUDA
using NNlibCUDA

src = CUDA.rand(2,3)

NNlib.gather(src,cu[1,4])

2×2 CuArray{Float32, 2, CUDA.Mem.DeviceBuffer}:
 0.430532  0.0
 0.474528  0.0
```