Internal library to share headers between test and bench

[achirkin]

Add a new component to raft for headers, which are used in both tests and benchmarks.

Closes #1153

[achirkin]

NB: this PR depend on and includes all changes in #1085

[achirkin]

I'm particularly not sure and welcome any suggestions regarding the naming of the component. Currently:

1. The name is raft_internal, because I wanted to call it internal, but it may be a reserved word in cmake.
2. All headers are placed into double-nested internal/internal. The first internal is for cmake to isolate it from the other components, the second is for includes, so that all includes from this component are easily identifiable, e.g.:

#include <internal/neighbors/naive_knn.cuh>

[cjnolet]

Thinking through this a little bit- what do you think about keeping the outer directory cpp/internal as-is but nesting everything inside cpp/internal/raft instead of cpp/internal/internal. This would effectively allows us to include the cpp/internal directory in the build for the bench and test source files whilst making the include paths themselves look the same as the other raft includes. So for example, this would still allow us to include the naive_knn.cuh file as #include <raft/neighbors/naive_knn.cuh> but the file would be coming from cpp/internal instead. This assumes we wouldn't have name collisions there, which I highly doubt we would.

Another option could be to do something like cpp/internal/raft/internal but I kind of prefer the option above more. What do you think?

[cjnolet]

The reason I'd prefer to have raft in the name is so that we don't have to worry about potential header collisions if, for example, a package named internal is ever shipped for linux. Separating the namespace by including raft makes it very clear where our scope ends.

[achirkin]

Actually, it turns out we have name collisions already! In branch-23.02 we have include/raft/matrix/select_k.cuh and its corresponding test test/raft/matrix/select_k.cuh; the latter is included using quotes syntax, so it's not a problem so far. In this PR I moved it to the internal component and changed the include syntax, so it would be a problem if we change the folder name to raft. So I see two options:

1. rename test/raft/matrix/select_k.cuh to something like select_k_helpers.cuh or select_k_utils.cuh
2. rename the root folder internal to something different from raft, yet not potentially clashing with third-party libraries.

In the new commit I opted for the option (2) with raft_internal as an experiment. Though I don't have a strong preference in this question. I only think that the trailing internal in the folder structure is not the best option, because it feels somewhat harder to maintain.

[cjnolet]

I think I can live w/ raft_internal in the meantime. We can always revisit if we find this is presenting maintenance challenges.

[codecov-commenter]

Codecov Report

Base: 87.99% // Head: 87.99% // No change to project coverage 👍

Coverage data is based on head (5cbd086) compared to base (7c12b1e).
Patch has no changes to coverable lines.

Additional details and impacted files

@@              Coverage Diff              @@
##           branch-23.02    #1162   +/-   ##
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  Coverage         87.99%   87.99%           
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  Files                21       21           
  Lines               483      483           
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  Hits                425      425           
  Misses               58       58           

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[cjnolet]

@achirkin I think this PR looks good. Can you also add a short description of this new directory to the bottom of the README.md in the root of the repository?

[cjnolet]

LGTM

[cjnolet]

/merge