Add scatter for CUDA support

[yuehhua]

Transfer from FluxML/NNlib.jl#296.

[yuehhua]

Wait for merge of JuliaGPU/CUDA.jl#842

[yuehhua]

Now this PR relies on NNlib#master (or next patch release of v0.7.19) and CUDA#master (or next patch release of v3.0.4).

[yuehhua]

@maleadt Could you trigger the CI test again? I think it is ready to go.
All tests passed on my PC.

[maleadt]

You can always amend and force-push to re-trigger CI.

[CarloLucibello]

You can also close and reopen to retrigger

[yuehhua]

I need at least NNlib v0.7.19 and CUDA v3.0.4 to pass CI, but buildkite just have the following version.

[052768ef] CUDA v3.0.0
[f6369f11] ForwardDiff v0.10.18
[872c559c] NNlib v0.7.18

As I know, there should be compatible with current version.

[maleadt]

Then you need to bump the versions in the Manifest. Buildkite doesn't use specific package versions.

[yuehhua]

Thank you. It works.

[CarloLucibello]

The compat bounds should also be updated

[yuehhua]

The compat bounds look somewhat like this?

[CarloLucibello]

And this other little piece is done, thank you for all these efforts!

[maleadt]

FWIW, it would have been better to squash and not merge this PR. Now there's 13 commits added to master for a single feature, many of which fail CI (complicating operations like git bisect).