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

Summary:
Pull Request resolved: https://github.com/facebookresearch/faiss/pull/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

tests/test_pq_code_distance.cpp

@@ -19,7 +19,11 @@
 
 #include <faiss/impl/FaissAssert.h>
 #include <faiss/impl/ProductQuantizer.h>
-#include <faiss/utils/pq_code_distance.h>
+#include <faiss/impl/pq_code_distance/pq_code_distance-inl.h>
+
+// Pull in inline NONE specializations so the test can call
+// PQCodeDistance<PQDecoder8, SIMDLevel::NONE> directly.
+#include <faiss/impl/pq_code_distance/pq_code_distance-generic.h>
 
 size_t nMismatches(
         const std::vector<float>& ref,
@@ -152,8 +156,8 @@ void test(
         for (size_t k = 0; k < 1000; k++) {
 #pragma omp parallel for schedule(guided)
             for (int64_t i = 0; i < (int64_t)n; i++) {
-                resultsDispatched1x[i] = faiss::pq_code_distance_single(
-                        subq, 8, lookup.data(), codes.data() + subq * i);
+                resultsDispatched1x[i] = faiss::pq_code_distance_8bit_single(
+                        subq, lookup.data(), codes.data() + subq * i);
             }
         }
         const auto endingTimepoint = std::chrono::steady_clock::now();
@@ -171,9 +175,8 @@ void test(
         for (size_t k = 0; k < 1000; k++) {
 #pragma omp parallel for schedule(guided)
             for (int64_t i = 0; i < (int64_t)n; i += 4) {
-                faiss::pq_code_distance_four(
+                faiss::pq_code_distance_8bit_four(
                         subq,
-                        8,
                         lookup.data(),
                         codes.data() + subq * (i + 0),
                         codes.data() + subq * (i + 1),