Remove AMDGPU and CUDA as direct dependencies

[James Schloss]

If we update to the latest version of KernelAbstractions, we should be able to remove CUDA, AMDGPU, ROCKernels, and CUDAKernels and instead use the KernelAbstractions get_backend(...) function, as in https://github.com/JuliaGPU/KernelAbstractions.jl/blob/main/examples/histogram.jl#L62

Note that this requires a slight rework of the layer.params.ArrayType interface as well because get_backend requires an actual array instead of an array type. Maybe just dispatch off of layer.canvas