[Add] Flatten Operator

Add C++ backend-free Flatten Operator with unit-tests for the Flatten_Op::forward function.

include/aidge/operator/Flatten.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_CORE_OPERATOR_FLATTEN_H_
+#define AIDGE_CORE_OPERATOR_FLATTEN_H_
+
+#include <memory>
+#include <vector>
+
+#include "aidge/backend/OperatorImpl.hpp"
+#include "aidge/graph/Node.hpp"
+#include "aidge/operator/OperatorTensor.hpp"
+#include "aidge/utils/Registrar.hpp"
+#include "aidge/utils/StaticAttributes.hpp"
+#include "aidge/utils/Types.h"
+
+namespace Aidge {
+class Flatten_OpImpl : public OperatorImpl {
+public:
+    Flatten_OpImpl(const Operator& op, const std::string& backend = ""): OperatorImpl(op, backend) {}
+    void forward() override;
+};
+
+enum class FlattenAttr { Axis };
+
+class Flatten_Op : public OperatorTensor,
+                   public Registrable<Flatten_Op, std::string, std::function<std::shared_ptr<OperatorImpl>(const Flatten_Op&)>> {
+
+public:
+    static const std::string Type;
+
+private:
+    using Attributes_ = StaticAttributes<FlattenAttr,
+                                            std::int64_t>;
+    template <FlattenAttr e> using attr = typename Attributes_::template attr<e>;
+    const std::shared_ptr<Attributes_> mAttributes;
+
+public:
+    Flatten_Op() = delete;
+
+    Flatten_Op(std::int64_t axis = 1);
+
+    /**
+     * @brief Copy-constructor. Copy the operator attributes and its output tensor(s), but not its input tensors (the new operator has no input associated).
+     * @param op Operator to copy.
+     */
+    Flatten_Op(const Flatten_Op& op);
+
+    /**
+     * @brief Clone the operator using its copy-constructor.
+     * @see Operator::Flatten_Op
+     */
+    std::shared_ptr<Operator> clone() const override;
+
+    bool forwardDims(bool allowDataDependency = false) override final;
+
+    void setBackend(const std::string& name, DeviceIdx_t device = 0) override final;
+    std::set<std::string> getAvailableBackends() const override;
+
+    std::shared_ptr<Attributes> attributes() const override { return mAttributes; }
+    inline std::int64_t& axis() const { return mAttributes->template getAttr<FlattenAttr::Axis>(); }
+
+    static const std::vector<std::string> getInputsName(){
+        return {"data_input"};
+    }
+    static const std::vector<std::string> getOutputsName(){
+        return {"data_output"};
+    }
+};
+
+std::shared_ptr<Node> Flatten(std::int64_t axis = 1,
+                            const std::string &name = "");
+}  // namespace Aidge
+
+namespace {
+template <>
+const char *const EnumStrings<Aidge::FlattenAttr>::data[] = { "axis" };
+}
+
+#endif /* AIDGE_CORE_OPERATOR_FLATTEN_H_ */