Remove unused variables in faiss/utils/partitioning.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: palmje

Differential Revision: D53779589

fbshipit-source-id: 2631d7b23f2bc79b0468a8d983f74547c15f9c15

faiss/utils/partitioning.cpp

@@ -206,7 +206,8 @@ typename C::T partition_fuzzy_median3(
         assert(n_eq_1 <= n_eq);
     }
 
-    int wp = compress_array<C>(vals, ids, n, thresh, n_eq_1);
+    [[maybe_unused]] const int wp =
+            compress_array<C>(vals, ids, n, thresh, n_eq_1);
 
     assert(wp == q);
     if (q_out) {