DD: give fvec_L2sqr_ny_nearest_y_transposed_D internal linkage (#5720)

Summary:
Pull Request resolved: https://github.com/facebookresearch/faiss/pull/5720

`fvec_L2sqr_ny_nearest_y_transposed_D<DIM>` is a template defined at namespace scope in both `distances_avx2.cpp` (`-mavx2`) and `distances_avx512.cpp` (`-mavx512*`), with the same signature, so both TUs emit the same weak symbols and the linker keeps one copy per `DIM`. Whichever copy it keeps is used by both levels. In a current DD build the AVX512 copies of `D<1>`, `D<2>`, `D<4>` were kept (72-95 `zmm` instructions each) and the AVX2 copy of `D<8>`, so the AVX512 path ran AVX2 code for `D<8>`. Nothing on the AVX2 path calls `D<1/2/4>` today, so there is no crash now; but a link that keeps the AVX512 `D<8>` would make the AVX2 path execute AVX512 instructions and raise SIGILL on CPUs without AVX512 (e.g. AMD Milan).

Same class as D124399620 (`PQCodeDistanceScalar`), found by scanning the DD binary for functions that contain `zmm` instructions but are not marked as an AVX512 level and have external linkage. The only other hits were `DCBF16_IP` / `DCBF16_L2`, which are defined only in `sq-avx512-spr.cpp` and reached only through AVX512_SPR dispatch.

Fix: put the template in an anonymous namespace in each TU, so each level keeps its own copy.

Reviewed By: mnorris11

Differential Revision: D124411046

fbshipit-source-id: 9165061e67f1476ff038c173aa837f7e0959e920

faiss/utils/simd_impl/distances_avx2.cpp

@@ -1060,6 +1060,11 @@ size_t fvec_L2sqr_ny_nearest<SIMDLevel::AVX2>(
             &fvec_L2sqr_ny_nearest_D8<SIMDLevel::AVX2>);
 }
 
+// Internal linkage: the same template is compiled with other -m flags in
+// the other per-SIMD distances TU, and a shared symbol would let the linker
+// hand one level the other's code.
+namespace {
+
 template <size_t DIM>
 size_t fvec_L2sqr_ny_nearest_y_transposed_D(
         float* /*distances_tmp_buffer*/,
@@ -1176,6 +1181,8 @@ size_t fvec_L2sqr_ny_nearest_y_transposed_D(
     return current_min_index;
 }
 
+} // namespace
+
 template <>
 size_t fvec_L2sqr_ny_nearest_y_transposed<SIMDLevel::AVX2>(
         float* distances_tmp_buffer,

faiss/utils/simd_impl/distances_avx512.cpp

@@ -1024,6 +1024,11 @@ size_t fvec_L2sqr_ny_nearest<SIMDLevel::AVX512>(
             &fvec_L2sqr_ny_nearest_D8<SIMDLevel::AVX512>);
 }
 
+// Internal linkage: the same template is compiled with other -m flags in
+// the other per-SIMD distances TU, and a shared symbol would let the linker
+// hand one level the other's code.
+namespace {
+
 template <size_t DIM>
 size_t fvec_L2sqr_ny_nearest_y_transposed_D(
         float* /* distances_tmp_buffer */,
@@ -1137,6 +1142,8 @@ size_t fvec_L2sqr_ny_nearest_y_transposed_D(
     return current_min_index;
 }
 
+} // namespace
+
 template <>
 size_t fvec_L2sqr_ny_nearest_y_transposed<SIMDLevel::AVX512>(
         float* distances_tmp_buffer,