Reduce memory usage of scalar index (#21970)

Signed-off-by: Nzhenshan.cao <zhenshan.cao@zilliz.com>

internal/core/src/index/IndexStructure.h

@@ -44,6 +44,6 @@ struct IndexStructure {
         return a_ == b.a_;
     }
     T a_;
-    size_t idx_;
+    int32_t idx_;
 };
 }  // namespace milvus::index

internal/core/src/index/ScalarIndexSort.h

@@ -81,7 +81,7 @@ class ScalarIndexSort : public ScalarIndex<T> {
  private:
     bool is_built_;
     Config config_;
-    std::vector<size_t> idx_to_offsets_;  // used to retrieve.
+    std::vector<int32_t> idx_to_offsets_;  // used to retrieve.
     std::vector<IndexStructure<T>> data_;
 };