Remove unused variables in faiss/impl/index_write.cpp

Summary:
LLVM-15 has a warning `-Wunused-but-set-variable` which we treat as an error because it's so often diagnostic of a code issue. Unused variables can compromise readability or, worse, performance.

This diff either (a) removes an unused variable and, possibly, it's associated code, or (b) qualifies the variable with `[[maybe_unused]]`, mostly in cases where the variable _is_ used, but, eg, in an `assert` statement that isn't present in production code.

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Reviewed By: algoriddle

Differential Revision: D52981052

fbshipit-source-id: 203e7ead56c69ffd77ee4a61eed1d0a502a14866

Author: r-barnes

faiss/impl/index_write.cpp

@@ -338,13 +338,11 @@ static void write_NSG(const NSG* nsg, IOWriter* f) {
     FAISS_THROW_IF_NOT(K == nsg->R);
     FAISS_THROW_IF_NOT(true == graph->own_fields);
 
-    int size = 0;
     for (int i = 0; i < N; i++) {
         for (int j = 0; j < K; j++) {
             int id = graph->at(i, j);
             if (id != EMPTY_ID) {
                 WRITE1(id);
-                size += 1;
             } else {
                 break;
             }