Expose kernel launch options as offload intrinsic args

[Sa4dUs]

Allows modifying the workgroup and thread grid dimensions directly from the intrinsic call.

core::intrinsics::offload(_kernel_1, [256, 1, 1], [32, 1, 1], (x,))

r? @ZuseZ4

[rustbot]

The rustc-dev-guide subtree was changed. If this PR only touches the dev guide consider submitting a PR directly to rust-lang/rustc-dev-guide otherwise thank you for updating the dev guide with your changes.

cc @BoxyUwU, @jieyouxu, @Kobzol, @tshepang

Some changes occurred to the intrinsics. Make sure the CTFE / Miri interpreter
gets adapted for the changes, if necessary.

cc @rust-lang/miri, @RalfJung, @oli-obk, @lcnr

[ZuseZ4]

@kevinsala it would be nice to have an example in our docs where giving the same dimensions as runtime value (e.g. via command line) vs giving them at compile time (like in the example here) have some measurable perf difference. I assume it's hard to artificially come up with an example exactly for that, or do you know how to get one?

[kevinsala]

@ZuseZ4 I'll try to find an example.

[ZuseZ4]

As per discussion with offload devs, this should be u32 not i32, otherwise lgtm for now.

Once we have a proper macro frontend for our intrinsic we can also consider changing it to [Option<NonZeroU32>;3], but until then, the interface would be way to cumbersome to call manually and [u32;3] is the best approximation. Certainly better than having the hardcoded values from one benchmark.

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.