Remove unused variables in faiss/IndexScalarQuantizer.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: D52981077

fbshipit-source-id: ad164f9fbe267c6f594f9298132578f0a3805a2c

faiss/IndexScalarQuantizer.cpp

@@ -210,12 +210,11 @@ void IndexIVFScalarQuantizer::add_core(
         const idx_t* coarse_idx) {
     FAISS_THROW_IF_NOT(is_trained);
 
-    size_t nadd = 0;
     std::unique_ptr<ScalarQuantizer::SQuantizer> squant(sq.select_quantizer());
 
     DirectMapAdd dm_add(direct_map, n, xids);
 
-#pragma omp parallel reduction(+ : nadd)
+#pragma omp parallel
     {
         std::vector<float> residual(d);
         std::vector<uint8_t> one_code(code_size);
@@ -240,7 +239,6 @@ void IndexIVFScalarQuantizer::add_core(
                 size_t ofs = invlists->add_entry(list_no, id, one_code.data());
 
                 dm_add.add(i, list_no, ofs);
-                nadd++;
 
             } else if (rank == 0 && list_no == -1) {
                 dm_add.add(i, -1, 0);