Added doc comment

src/operator/MetaOperator.cpp

@@ -73,6 +73,9 @@ Aidge::MetaOperator_Op::MetaOperator_Op(const MetaOperator_Op& op)
 std::shared_ptr<Aidge::Operator> Aidge::MetaOperator_Op::clone() const {
     auto metaOp = std::make_shared<MetaOperator_Op>(*this);
     if (mImpl) {
+        // Only setBackend() is mImpl is not nullptr.
+        // The inner-graph backend is already set in MetaOperator_Op copy
+        // construtor, when the graph is cloned.
         metaOp->setBackend(mImpl->backend());
     }
     return metaOp;