Minor speedup of HNSW (facebookresearch#2832)

Summary:
Pull Request resolved: facebookresearch#2832

Switch to `omp schedule(guided)` in HNSW::search() call

Reviewed By: mdouze

Differential Revision: D45360510

fbshipit-source-id: 5230429748c0e1e12d8fc2ea7184e273dee76bdf

Author: Alexandr Guzhva

faiss/IndexHNSW.cpp

@@ -312,7 +312,7 @@ void IndexHNSW::search(
             DistanceComputer* dis = storage_distance_computer(storage);
             ScopeDeleter1<DistanceComputer> del(dis);
 
-#pragma omp for reduction(+ : n1, n2, n3, ndis, nreorder)
+#pragma omp for reduction(+ : n1, n2, n3, ndis, nreorder) schedule(guided)
             for (idx_t i = i0; i < i1; i++) {
                 idx_t* idxi = labels + i * k;
                 float* simi = distances + i * k;