Implement `KernelAbstractions.get_backend(::Memory{Float64})`

[FerreolS]

Hi,
KernelAbstractions is really awesome to get things work fast on both CPU and GPU without bothering too much.
However, I'm missing a get_backend for Memory type.
I suppose it is just a matter of adding somewhere

get_backend(::Memory) = CPU()

I can even make a PR for that, am I missing something?

Actually, I need that for FixedSizeArrays where parent(::FixedSizeArray) return a Memory array. I can also propose an extension there but I believe it is better to consider the Memory case here.
Thanks

[vchuravy]

We can add a definition for Memory, but the usual way is to have an extension

[FerreolS]

The thing is that Memory is in Core. I can't really make an extension for Core. Adding this definition elsewhere will be type piracy.