Optimization of ScatterElementsUpdate ref impl (#18313)

Co-authored-by: Katarzyna Mitrus <katarzyna.mitrus@intel.com>
Co-authored-by: Michal Lukaszewski <michal.lukaszewski@intel.com>

src/core/reference/include/ngraph/runtime/reference/scatter_elements_update.hpp

@@ -149,10 +149,7 @@ typename std::enable_if<std::is_floating_point<T>::value || std::is_class<T>::va
 
 template <typename T>
 typename std::enable_if<std::is_integral<T>::value, T>::type arithmetic_mean(const T accumulator, const int32_t N) {
-    const auto old_mode = std::fegetround();
-    std::fesetround(FE_DOWNWARD);
     const T value = static_cast<T>(std::nearbyint(static_cast<double>(accumulator) / N));
-    std::fesetround(old_mode);
     return value;
 }
 
@@ -165,6 +162,25 @@ size_t normalize_index(const T idx, const size_t dim_value) {
     }
 }
 
+template <typename T>
+struct RoundingDirectionGuard {
+    RoundingDirectionGuard() {
+        if (std::is_integral<T>::value) {
+            m_original_mode = std::fegetround();
+            std::fesetround(FE_DOWNWARD);
+        }
+    }
+
+    ~RoundingDirectionGuard() {
+        if (std::is_integral<T>::value) {
+            std::fesetround(m_original_mode);
+        }
+    }
+
+private:
+    decltype(std::fegetround()) m_original_mode;
+};
+
 template <typename DataType, typename IndicesType>
 void scatter_elem_update_with_reduction(const DataType* input_data,
                                         const IndicesType* indices,
@@ -221,6 +237,9 @@ void scatter_elem_update_with_reduction(const DataType* input_data,
     }
 
     if (reduction_type == Reduction::MEAN) {
+        // this object will change the rounding mode only for integer types which is required to match torch
+        // upon destruction the previously used rounding mode will be restored
+        RoundingDirectionGuard<DataType> rounding_guard;
         for (const auto& counter : mean_reduction_counters) {
             // include the initial value in the arithmetic mean divisor (if needed)
             const auto N = counter.second + static_cast<int32_t>(use_init_val);