Improved fuseToMetaOps to set backend of fused meta op

Partially fixes #163 (closed).

Note that currently, nested meta-ops scheduling is not well supported!

include/aidge/operator/MetaOperator.hpp

@@ -79,6 +79,7 @@ public:
         return false;
     }
 
+    std::string backend() const noexcept override;
 
     void setBackend(const std::string &name, DeviceIdx_t device = 0) override;