HNSW CPU Hierarchy

[divyegala]

This PR adds an option to build the full HNSW hierarchy on the CPU when converting a CAGRA index to an hnswlib index. It also updates hnswlib to v0.7.0

[jakirkham]

Does this need to be added to the Conda recipes as well?

[divyegala]

@jakirkham no actually we don't want to use the hnswlib conda package because their python distribution messes up with our python version requirements. I will be removing it from our conda dev environments as well.

We'll rely on CMake instead to get us hnswlib. There's no double compilation issue anyway since it's a header-only library.

[jakirkham]

Oh ok. Had not looked into the nuances of this package or our usage of it

Indeed if we don't use the Python package, we should drop it

There doesn't appear to be a header-only C++ package currently. So just fetching it from source seems like the thing to do

Are the headers just used in the build to produce a library? Or are we shipping their headers too? If the latter, we may want to look at siloing it somehow to avoid clobbering or affecting other installs a user may have

[divyegala]

@jakirkham we don't need to ship the headers, they are self-contained within the libcuvs shared object.