Add a Backward Pass for the MetaOperators

Description

The metaOperator class was lacking an implementation for the backward pass. This MR address this.

The code is based on the implementation of the MetaOperator's forward pass.

Modified files : MetaOperator.cpp, MetaOperator.hpp

include/aidge/operator/MetaOperator.hpp

@@ -203,13 +203,8 @@ public:
 
     /**
      * @brief Perform the backward pass for the operator.
-     * 
-     * @note Currently not implemented.
      */
-    void backward() override {
-        AIDGE_THROW_OR_ABORT(std::runtime_error, "backward() not implemented yet for a MetaOperator");
-    }
-
+    void backward() override;
     /**
      * @brief Check if the operator is atomic.
      *