Use uint8_t instead of uint32_t for faiss::VisitedTable.visno (facebookresearch#2873)

Summary: Pull Request resolved: facebookresearch#2873

Reviewed By: mdouze

Differential Revision: D46125491

fbshipit-source-id: 9c48bb55e54eb361438521494093a3f9ab823857

Author: Alexandr Guzhva

faiss/impl/AuxIndexStructures.h

@@ -165,7 +165,7 @@ struct FAISS_API InterruptCallback {
 /// set implementation optimized for fast access.
 struct VisitedTable {
     std::vector<uint8_t> visited;
-    int visno;
+    uint8_t visno;
 
     explicit VisitedTable(int size) : visited(size), visno(1) {}