Minor AbstractQ audit

[dkarrasch]

Fixes part of JuliaGPU/CUDA.jl#1866, resolves #46196 (comment). I removed some not exported functions that hadn't existed before #46196 anyway and added a few tests.

[maleadt]

Thanks!

[dkarrasch]

Have you checked, does this fix the entire GPU package stack? Or will it be easier to merge this as is and then see what happens with nightly?

[dkarrasch]

Let's go with this and then fix issues as/if they arise.

[maleadt]

Sorry, didn't have time to test this over the weekend.
It fixes the immediate issue, but it looks like #46196 has broken other parts of CUDA.jl. For example, our tests were relying on collect(::QRCompactWYQ) to get an Array, but that doesn't seem to work anymore post #46196. I assume that's intended, as collect is documented to get an Array from anything iterable?

[dkarrasch]

I see. I've "fixed" quite a couple of packages with this usage. If you're fine with the result being a plain Matrix, then replace collect by Matrix, or replace collect(Q) by Q*I (i.e., the UniformScaling I). That might be able to return a CuArray, if that's what the corresponding Q * true does. I hope this is just a test pattern, not a productive code pattern.

I don't think I can develop CUDA.jl locally, so should I just fork the repo, start a PR and test with online CI until it's all green?