Suppress unused static constexpr variable warnings (#5087)

Summary:
Pull Request resolved: #5087

Fix 2 `clang-diagnostic-unused-const-variable` lint warnings by adding `[[maybe_unused]]` to `static constexpr int impl` in `IndexIVFPQFastScan.cpp` and `IndexIVFRaBitQFastScan.cpp`. The variable serves as documentation about the search implementation being used, so it is kept rather than removed.

Reviewed By: mnorris11

Differential Revision: D100575853

fbshipit-source-id: 1c42958b1de1b663dc8f2eadfff345d27583b8a2

Author: junjieqi

faiss/IndexIVFPQFastScan.cpp

@@ -319,7 +319,8 @@ namespace {
 
 struct IVFPQFastScanScanner : InvertedListScanner {
     using InvertedListScanner::scan_codes;
-    static constexpr int impl = 10; // based on search_implem_10
+    [[maybe_unused]] static constexpr int impl =
+            10;                     // based on search_implem_10
     static constexpr size_t nq = 1; // 1 query at a time.
     const IndexIVFPQFastScan& index;
     AlignedTable<uint8_t> dis_tables;

faiss/IndexIVFRaBitQFastScan.cpp

@@ -573,7 +573,7 @@ namespace {
 /// Provides IVF scanner interface using FastScan's SIMD batch processing.
 struct IVFRaBitQFastScanScanner : InvertedListScanner {
     using InvertedListScanner::scan_codes;
-    static constexpr int impl = 10;
+    [[maybe_unused]] static constexpr int impl = 10;
     static constexpr size_t nq = 1;
 
     const IndexIVFRaBitQFastScan& index;