Inline PQ code distance kernels into scanner TUs (#5159)

Summary:
Pull Request resolved: #5159

After the SIMD dispatch refactoring, the PQ code distance implementations
(pq_code_distance_single_impl<SL>, pq_code_distance_four_impl<SL>) lived in
separate translation units from the scanner loops (IVFPQScannerT::scan_list_with_table,
PQDistanceComputer::distance_to_code). The compiler could not inline the SIMD
gather/accumulate code into the hot inner loops.

This diff converts the per-SIMD pq_code_distance .cpp files to .h headers and
includes them in the corresponding scanner TUs before the scanner _impl.h
includes. This puts the kernel definitions in the same TU, enabling the compiler
to inline the AVX2/AVX512 vgatherdps code directly into scan_list_with_table and
scan_list_polysemous_hc.

Changes:
- pq_code_distance-avx2.cpp → pq_code_distance-avx2.h (header, #pragma once)
- pq_code_distance-avx512.cpp → pq_code_distance-avx512.h (same)
- New pq_code_distance-generic.h with inline NONE/ARM_NEON specializations
- Scanner TUs (avx2.cpp, avx512.cpp, neon.cpp) include the PQ distance headers
- PQCodeDistance gains static constexpr simd_level member
- scan_list_polysemous uses PQCodeDist::simd_level for Hamming computer dispatch
  (was hardcoded to SIMDLevel::NONE)
- Scanner TUs include per-ISA hamming_computer headers for SIMD Hamming dispatch
- Build files (xplat.bzl, CMakeLists.txt) updated

Verified via objdump that scan_list_with_table and scan_list_polysemous_hc contain
zero calls to pq_code_distance_*_impl — all AVX2/AVX512 gather code is fully inlined.

Reviewed By: algoriddle

Differential Revision: D102942787

fbshipit-source-id: d2979f9cd629652ac1de4886afd3c335fbb4fac8

Author: mdouze

faiss/CMakeLists.txt

@@ -11,7 +11,7 @@
 set(FAISS_SIMD_AVX2_SRC
   impl/fast_scan/impl-avx2.cpp
   impl/hnsw/avx2.cpp
-  impl/pq_code_distance/pq_code_distance-avx2.cpp
+  impl/pq_code_distance/avx2.cpp
   impl/scalar_quantizer/sq-avx2.cpp
   impl/approx_topk/avx2.cpp
   impl/binary_hamming/avx2.cpp
@@ -25,7 +25,7 @@ set(FAISS_SIMD_AVX2_SRC
 set(FAISS_SIMD_AVX512_SRC
   impl/fast_scan/impl-avx512.cpp
   impl/hnsw/avx512.cpp
-  impl/pq_code_distance/pq_code_distance-avx512.cpp
+  impl/pq_code_distance/avx512.cpp
   impl/scalar_quantizer/sq-avx512.cpp
   impl/binary_hamming/avx512.cpp
   utils/simd_impl/distances_avx512.cpp
@@ -39,6 +39,7 @@ set(FAISS_SIMD_NEON_SRC
   impl/scalar_quantizer/sq-neon.cpp
   impl/approx_topk/neon.cpp
   impl/binary_hamming/neon.cpp
+  impl/pq_code_distance/neon.cpp
   utils/simd_impl/distances_aarch64.cpp
   utils/hamming_distance/hamming_neon.cpp
   utils/simd_impl/partitioning_neon.cpp
@@ -133,6 +134,7 @@ set(FAISS_SRC
   impl/PolysemousTraining.cpp
   impl/ProductQuantizer.cpp
   impl/pq_code_distance/pq_code_distance-generic.cpp
+  impl/pq_code_distance/IVFPQ_QueryTables.cpp
   impl/AdditiveQuantizer.cpp
   impl/RaBitQuantizer.cpp
   impl/RaBitQuantizerMultiBit.cpp
@@ -311,8 +313,13 @@ set(FAISS_HEADERS
   impl/simd_dispatch.h
   impl/fast_scan/simd_result_handlers.h
   impl/zerocopy_io.h
-  utils/pq_code_distance.h
   impl/pq_code_distance/pq_code_distance-inl.h
+  impl/pq_code_distance/pq_code_distance-avx2.h
+  impl/pq_code_distance/pq_code_distance-avx512.h
+  impl/pq_code_distance/pq_code_distance-generic.h
+  impl/pq_code_distance/IVFPQ_QueryTables.h
+  impl/pq_code_distance/IVFPQScanner_impl.h
+  impl/pq_code_distance/PQDistanceComputer_impl.h
   invlists/BlockInvertedLists.h
   invlists/DirectMap.h
   invlists/InvertedLists.h