Refactor OperatorImpl for backend/export

Refactor OperatorImpl for backend/export.

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

@@ -38,7 +38,7 @@ public:
         return impl.prodConso(mOp);
     }
 
-    virtual std::vector<ImplSpec> getAvailableImplSpecs() const override {
+    virtual std::set<ImplSpec> getAvailableImplSpecs() const override {
         return Registrar<OperatorImpl_cpu>::getKeys();
     }