Add LRN operator

Add LRN operator, which is a bit legacy, but was there in N2D2 and is really cheap to add (and still used in some ONNX models).

Fix GlobalAveragePooling unit test following aidge_core#195 (closed).

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

+/********************************************************************************
+ * Copyright (c) 2023 CEA-List
+ *
+ * This program and the accompanying materials are made available under the
+ * terms of the Eclipse Public License 2.0 which is available at
+ * http://www.eclipse.org/legal/epl-2.0.
+ *
+ * SPDX-License-Identifier: EPL-2.0
+ *
+ ********************************************************************************/
+
+#ifndef AIDGE_CPU_OPERATOR_LRNIMPL_H_
+#define AIDGE_CPU_OPERATOR_LRNIMPL_H_
+
+#include "aidge/backend/cpu/operator/OperatorImpl.hpp"
+#include "aidge/operator/LRN.hpp"
+#include "aidge/utils/Registrar.hpp"
+#include "aidge/utils/Types.h"
+#include "aidge/backend/cpu/data/GetCPUPtr.h"
+#include <memory>
+#include <vector>
+
+namespace Aidge {
+// Operator implementation entry point for the backend
+using LRNImpl_cpu = OperatorImpl_cpu<LRN_Op,
+    void(float, float, float, std::size_t, const std::vector<DimSize_t>&, const void*, void*)>;
+
+// Implementation entry point registration to Operator
+REGISTRAR(LRN_Op, "cpu", Aidge::LRNImpl_cpu::create);
+}  // namespace Aidge
+
+#endif /* AIDGE_CPU_OPERATOR_LRNIMPL_H_ */