Prefer IVF-PQ compute_similarity blockDim=512 on AMD (gfx1100)

[konkolchin]

Summary

• Prefer blockDim=512 for IVF-PQ compute_similarity launches on HIP/AMD after stock occupancy auto-shrink, which under-sizes blocks on RDNA3 (gfx1100).
• Measured on RX 7900 XTX (library IVF_PQ m=32, SIFT-1M): ~+21–26% search QPS at nprobe 8/16/32 vs stock heuristic, recall@10 unchanged; optional env HIPVS_IVF_PQ_BLOCK_THREADS still overrides when set.
• Mid-grid hipVS/cuVS library ratio improves from ~0.48–0.50× to ~0.60–0.62× on peer GPUs (not a claim of closing the full gap).

Context

• Validated in DXC/AMD PoC harness: https://github.com/konkolchin/ann-harness-amd (results/lib_bench/LAUNCH_KNOBS.md, FILLED_pq_m32.md).
• Change is HIP-only (__HIP_PLATFORM_AMD__) inside compute_similarity_select.

Test plan

• Library bench on gfx1100: INDEX_TYPE=IVF_PQ M=32 — stderr shows block_threads=512; nprobe=32 QPS ~270k vs ~217k stock
• Recall@10 ladder unchanged (~0.35 → ~0.73)
• CI / reviewers: build libcuvs for target AMD arch and run existing IVF-PQ gtests / neighbors benches if available