Nit: fix variable types (#3147)

Summary:
The inconsistency in variable types would generate waaaaaaay too many warnings `comparison of integer expressions of different signedness` during the compilation if all the warnings are made reported

Pull Request resolved: #3147

Reviewed By: mlomeli1

Differential Revision: D51988074

Pulled By: algoriddle

fbshipit-source-id: 31a57bbe3a14105a968d3f06c0807c2fe60f1a62

Author: alexanderguzhva

faiss/impl/pq4_fast_scan_search_1.cpp

@@ -125,7 +125,7 @@ void accumulate_fixed_blocks(
         ResultHandler& res,
         const Scaler& scaler) {
     constexpr int bbs = 32 * BB;
-    for (int64_t j0 = 0; j0 < nb; j0 += bbs) {
+    for (size_t j0 = 0; j0 < nb; j0 += bbs) {
         FixedStorageHandler<NQ, 2 * BB> res2;
         kernel_accumulate_block<NQ, BB>(nsq, codes, LUT, res2, scaler);
         res.set_block_origin(0, j0);

faiss/impl/pq4_fast_scan_search_qbs.cpp

@@ -123,7 +123,7 @@ void accumulate_q_4step(
     constexpr int Q4 = (QBS >> 12) & 15;
     constexpr int SQ = Q1 + Q2 + Q3 + Q4;
 
-    for (int64_t j0 = 0; j0 < ntotal2; j0 += 32) {
+    for (size_t j0 = 0; j0 < ntotal2; j0 += 32) {
         FixedStorageHandler<SQ, 2> res2;
         const uint8_t* LUT = LUT0;
         kernel_accumulate_block<Q1>(nsq, codes, LUT, res2, scaler);
@@ -156,7 +156,7 @@ void kernel_accumulate_block_loop(
         const uint8_t* LUT,
         ResultHandler& res,
         const Scaler& scaler) {
-    for (int64_t j0 = 0; j0 < ntotal2; j0 += 32) {
+    for (size_t j0 = 0; j0 < ntotal2; j0 += 32) {
         res.set_block_origin(0, j0);
         kernel_accumulate_block<NQ, ResultHandler>(
                 nsq, codes + j0 * nsq / 2, LUT, res, scaler);
@@ -243,7 +243,7 @@ void pq4_accumulate_loop_qbs(
 
     // default implementation where qbs is not known at compile time
 
-    for (int64_t j0 = 0; j0 < ntotal2; j0 += 32) {
+    for (size_t j0 = 0; j0 < ntotal2; j0 += 32) {
         const uint8_t* LUT = LUT0;
         int qi = qbs;
         int i0 = 0;