Better errors for un-implemented functions

[mcabbott]

Functions like gather/scatter give scalar indexing errors if used on CuArrays without remembering to load NNlibCUDA.

Since there is now a very lightweight GPUArraysCore, I think NNlib should depend on that, and define dumb methods on ::AbstractGPUArray which throw a helpful error.

Edit, 2023:

After #492 we use extensions. If I understand right it may still be possible to forget to load cuDNN.

Some kernels don't yet work for Metal / AMDGPU, e.g. FluxML/Flux.jl#2278 . Here too it would be nicer to get a clear "not implemented" error.