Add log operator for BCE loss function

Context

To be able to compute Binary Cross Entropy (BCE) using a graph, log operator is needed.

Modified files

The operator "Ln" is created for CPU backend.

include/aidge/backend/cpu/operator/LnImpl.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_LNIMPL_H_
+#define AIDGE_CPU_OPERATOR_LNIMPL_H_
+
+#include "aidge/backend/OperatorImpl.hpp"
+#include "aidge/operator/Ln.hpp"
+#include "aidge/utils/Registrar.hpp"
+#include "aidge/utils/Types.h"
+#include "aidge/backend/cpu/data/GetCPUPtr.h"
+#include <memory>
+#include <vector>
+
+namespace Aidge {
+// class Ln_Op;
+
+// compute kernel registry for forward and backward
+class LnImplForward_cpu
+    : public Registrable<LnImplForward_cpu, std::tuple<DataType, DataType>, void(const std::size_t, const void*, void*)> {
+};
+class LnImplBackward_cpu
+    : public Registrable<LnImplBackward_cpu, std::tuple<DataType, DataType, DataType>, void(const std::size_t, const void*, const void*, void*)> {
+};
+
+class LnImpl_cpu : public OperatorImpl {
+public:
+    LnImpl_cpu(const Ln_Op& op) : OperatorImpl(op, "cpu") {}
+
+    static std::unique_ptr<LnImpl_cpu> create(const Ln_Op& op) {
+        return std::make_unique<LnImpl_cpu>(op);
+    }
+
+    Elts_t getNbRequiredProtected(const IOIndex_t inputIdx) const override final;
+	
+    void forward() override final;
+
+    void backward() override final;
+};
+
+namespace {
+static Registrar<Ln_Op> registrarLnImpl_cpu("cpu", Aidge::LnImpl_cpu::create);
+}
+}  // namespace Aidge
+
+#endif /* AIDGE_CPU_OPERATOR_LNIMPL_H_ */