Fea 2208 kmeans use specializations

[cjnolet]

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[cjnolet]

@Nyrio just FYI, I did realize after some investigation that this doesn't fix the horrid build time of k-means because the standard distance instantiations seem to be getting ignored- telling me k-means must be using a template combination that we're not instantiating. I haven't investigated exactly which ones it's using yet, though.

[Nyrio]

@cjnolet I've compiled the kmeans test with your changes and then run nm and this is what I found:

• 28/32 of the raft:distance::detail::distance are marked as "U" i.e undefined in this file (correctly identified as extern)
• The 4 remaining correspond to a metric that hasn't been added to the specializations, as I pointed in a comment on my PR
• 20 instances of fusedL2NNkernel are recompiled every time.

I think your PR is working. Horrid compilation times are due to fusedL2NN, which is why I made a specialization for it in #795

[cjnolet]

Perfect and thanks for investigating this! If you'd like, we can get this PR merged ASAP and then add the specializations for the missing metrics in your PR. Or I can add them here, whichever you prefer.

[Nyrio]

I think we can merge this one, and I'll add the rest to my PR.

[cjnolet]

@gpucibot merge