Use HNSW GPU Hierarchy by Default

[julianmi]

Change the default HNSW hierarchy from NONE to GPU to enable search with Hnswlib. Searching an HNSW index built with HnswHierarchy::NONE using Hnswlib fails with a segmentation fault. This change should go into 25.12 to prevent such issues and improve the interoperability between cuVS and Hnswlib.

[tfeher]

Thanks Julian for fixing this, LGTM!

[divyegala]

@tfeher this is a breaking change, and it's also missing an update to the C API default.

[divyegala]

Instead of a segmentation fault, can you put an error checker to fail gracefully if user tries to search a NONE hierarchy HNSW index?

[julianmi]

@tfeher this is a breaking change, and it's also missing an update to the C API default.

I have changed the C API default as well.

Instead of a segmentation fault, can you put an error checker to fail gracefully if user tries to search a NONE hierarchy HNSW index?

Sorry to not me more specific. Our internal search works but a separate search with Hnswlib fails. Changing the default would enable the interoperability out of the box.

[tfeher]

@tfeher this is a breaking change

Thanks @divyegala for catching my mistake. Indeed, users might be relying on the default value, so this is a breaking change. I have updated the labels.