fix ReduceMean type

include/aidge/operator/ReduceMean.hpp

@@ -132,6 +132,10 @@ inline std::shared_ptr<Node> ReduceMean(
     static_assert(DIM<=MaxDim,"Too many kernel dimensions required by ReduceMean, not supported");
     return ReduceMean(to_array(axes), keep_dims, name);
 }
+
+template <DimIdx_t DIM>
+const std::string ReduceMean_Op<DIM>::Type = "ReduceMean";
+
 }  // namespace Aidge
 
 namespace {