Moving MG functions into unified API + raft::device_resources_snmg as device resource type for MG functions

[viclafargue]

• Moving MG functions into unified API
• Using raft::device_resources_snmg as device resource type for MG functions
• Removing instantiation of raft::comms::build_comms_nccl_only solving [FEA] Remove dependency of build_comms_nccl_only on UCP raft#2465

raft::device_resources_snmg is intoroduced in this PR : rapidsai/raft#2549

[cjnolet]

One of the main goals with having the "snmg" resources object match the single-gpu object is that we wanted to be able to remove this additional MG. We should now be able to accept device_resources and then check to see if a nccl clique has been set on it (which would imply that it's a multi-gpu resources object and not a single-gpu object). The whole goal with doing this was to consolidate code paths.

[viclafargue]

The NCCL clique is not set as a resource anymore, but we should still be able to implement the dispatching by checking the dynamic type of the device_resources. The real question then is, do we truly want dispatching on both the regular API (cuvs::neighbors::build) and the mg namespace (cuvs::neighbors::mg::build)? It kind of make sense that a user providing a device_resources_snmg instance to the regular API (cuvs::neighbors::build) would want things to be deployed on multiple GPUs. However, the reverse is not necessarily true. A user who explicitly chose the mg namespace, but did not provide the adequate device_resources_snmg would fallback to single GPU, potentially unintentionally. Is this what we want?

I propose that we implement the dispatching mechanism solely on the regular API (cuvs::neighbors::build) in a dedicated follow-up PR? This also allows the MG doc to explicitly avert users that they should use an adequate device_resources_snmg to use the MG API. What do you think?

[cjnolet]

Not sure why CI is still failing. The RAFT PR was merged quite awhile ago but I'm still seeing errors like this:

/home/coder/cuvs/cpp/include/cuvs/neighbors/mg.hpp:24:10: fatal error: raft/core/device_resources_snmg.hpp: No such file or directory
     24 | #include <raft/core/device_resources_snmg.hpp>
        |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

[bdice]

This fell back to old 24.12 RAFT packages for unclear reasons:

    libraft-headers:             24.12.00-cuda12_241211_geaf9cc72_0 rapidsai   
    libraft-headers-only:        24.12.00-cuda12_241211_geaf9cc72_0 rapidsai   

https://github.com/rapidsai/cuvs/actions/runs/12818869804/job/35749392606?pr=454#step:8:521

[cjnolet]

This fell back to old 24.12 RAFT packages for unclear reasons:

Oh fooey! Thanks for noticing that @bdice, I just realized the darn target branch was never updated!

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