add a backward pass for the metaoperators

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.
      * 

src/operator/MetaOperator.cpp

@@ -273,6 +273,21 @@ void Aidge::MetaOperator_Op::forward() {
     }
 }
 
+void Aidge::MetaOperator_Op::backward() {
+    if (mImpl) {
+        // A custom implementation exists for this meta operator
+        mImpl->backward();
+    }
+    else {
+        // No custom implementation, use the individual operators implementations
+        if (!mScheduler) {
+            mScheduler = std::make_shared<SequentialScheduler>(mGraph, mUpperNode.lock());
+            mScheduler->generateScheduling();
+        }
+        mScheduler->backward();
+    }
+}
+
 /////////////////////////////////////////////////
 
 std::shared_ptr<Aidge::Node> Aidge::MetaOperator(const char *type,