feat: GPU_HNSW index using faiss::gpu::GpuIndexHNSW (slim, feature-only)

[devin-ai-integration]

Summary

Slim, review-friendly version of the GPU_HNSW feature currently on gpu-hnsw-faiss (PR #2).

The gpu-hnsw-faiss branch mixes the feature with a full upstream sync (faiss upgrade + SINDI/sparse) via a Merge upstream/main commit, so its diff-vs-main is ~19.5k lines / 189 files. This branch rebases only the feature onto current main and squashes the noisy development history (perf experiments + their reverts) into one commit, so the diff is exactly the feature:

src/index/hnsw/faiss_hnsw.cc                       (GPU_HNSW IndexNode wiring)
include/knowhere/comp/index_param.h                (GPU_HNSW index type)
include/knowhere/index/index_table.h
cmake/libs/lib{faiss,diskann}.cmake                (faiss_gpu_hnsw target)
tests/ut/test_gpu_search.cc                        (recall / cosine / topk / serialize tests)
thirdparty/faiss/faiss/gpu/**                      (vendored GpuIndexHNSW kernel)
→ 14 files, +2559 / -2

No upstream sync, no reverted char4/dp4a experiments — just the final feature.

Relationship to other PRs:

• Supersedes the reviewability of feat: GPU_HNSW / GPU_HNSW_SQ index using faiss::gpu::GpuIndexHNSW — native FP32/FP16/BF16/INT8 #2 (gpu-hnsw-faiss → main). If we prefer to keep landing via gpu-hnsw-faiss, this branch is still useful as the clean diff to review against.
• The dead-code removal in refactor: remove dead cooperative-distance code from GPU HNSW kernel #4 (cooperative-distance block in the vendored kernel) is not included here yet — this branch is a faithful slim of the current feature; refactor: remove dead cooperative-distance code from GPU HNSW kernel #4 can be applied on top or folded in on request.

Not build-verified locally in this session; tree is byte-identical to gpu-hnsw-faiss^1.

Link to Devin session: https://6sense.devinenterprise.com/sessions/d39aba56b8a3467cbbf231ab631a06ed

[devin-ai-integration]

🤖 Devin AI Engineer

I'll be helping with this pull request! Here's what you should know:

✅ I will automatically:

• Address comments on this PR. Add '(aside)' to your comment to have me ignore it.
• Look at CI failures and help fix them

Note: I can only respond to comments from users who have write access to this repository.

⚙️ Control Options:

• Disable automatic comment, CI, and merge conflict monitoring

[devin-ai-integration]

Superseded: this content is now folded directly into the clean gpu-hnsw-faiss branch (rebased onto synced upstream master, dead-code/GPU_TIMING cleanups included). Closing.