Fix convertion error

include/aidge/backend/cpu/operator/ReduceMeanImpl_forward_kernels.hpp

@@ -68,7 +68,8 @@ void ReduceMeanImpl_cpu_forward_kernel(const typename ReduceMean_Op<DIM>::Attrs&
         const I* inputAccumulation = input;
         I* outputAccumulation = nullptr;
 
-        for (const std::size_t& a : std::get<0>(attrs)) {
+        for (const auto& axisInt : std::get<0>(attrs)) {
+            const std::size_t a = static_cast<std::size_t>(axisInt);
             outputElements /= inputDims[a];
             outputAccumulation = new I[outputElements];
             const std::size_t dim_i = inputDims[a];