[Add] Heaviside Operator

Context

This will be useful for SNNs, as the relationship between spiking S and potential U can be described as :

S[t] = \Theta(U[t] - U_{\rm thr}) where \Theta(\cdot) is the Heaviside step function.

See also pytorch operator

Fixes #173.

include/aidge/operator/Heaviside.hpp

+/********************************************************************************
+ * Copyright (c) 2024 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_HEAVISIDE_H_
+#define AIDGE_CORE_OPERATOR_HEAVISIDE_H_
+
+#include <memory>
+#include <string>
+#include <vector>
+
+#include "aidge/backend/OperatorImpl.hpp"
+#include "aidge/graph/Node.hpp"
+#include "aidge/operator/Operator.hpp"
+#include "aidge/operator/OperatorTensor.hpp"
+#include "aidge/utils/Registrar.hpp"
+#include "aidge/utils/StaticAttributes.hpp"
+#include "aidge/utils/Types.h"
+
+namespace Aidge {
+enum class HeavisideAttr { 
+    /**
+     * @brief The value used in the output tensor when the input is 0.
+     */
+    Value 
+};
+
+class Heaviside_Op
+    : public OperatorTensor,
+      public Registrable<
+          Heaviside_Op,
+          std::string,
+          std::function<std::shared_ptr<OperatorImpl>(const Heaviside_Op &)>> {
+  private:
+    using Attributes_ = StaticAttributes<HeavisideAttr, float>;
+    template <HeavisideAttr e>
+    using attr = typename Attributes_::template attr<e>;
+    const std::shared_ptr<Attributes_> mAttributes;
+
+  public:
+    static const std::string Type;
+
+    /*
+     * Compute the Heaviside step function for each element of the first input.
+     * Heaviside step function is defined as :
+     *
+     * \f[
+     * heaviside(input, values) = \begin{cases}
+     *     0      & \text{if input }  < 0 \\
+     *     values & \text{if input }  = 0 \\
+     *     1      & \text{if input }  > 0
+     * \end{cases}
+     * \f]
+     * */
+    Heaviside_Op(float value);
+
+    Heaviside_Op(const Heaviside_Op &op);
+
+    std::shared_ptr<Operator> clone() const override;
+
+    void setBackend(const std::string &name, DeviceIdx_t device = 0) override;
+
+    std::set<std::string> getAvailableBackends() const override;
+
+    static const std::vector<std::string> getInputsName() {
+        return {"data_input", "data_values"};
+    }
+
+    static const std::vector<std::string> getOutputsName() {
+        return {"output"};
+    }
+
+    inline std::shared_ptr<Attributes> attributes() const override {
+        return mAttributes;
+    }
+    inline float &value() const {
+        return mAttributes->template getAttr<HeavisideAttr::Value>();
+    }
+};
+
+/**
+ * @brief Create a Heaviside node.
+ *
+ * Initializes a Heaviside node that computes the Heaviside step function for each element 
+ * of the input tensor, using the specified value for inputs equal to zero.
+ *
+ * @param value The value used in the output tensor when the input is 0.
+ * @param name  Optional. The name of the node.
+ * 
+ * @return A shared pointer to a Node representing the Heaviside operation.
+ */
+std::shared_ptr<Node> Heaviside(float value, const std::string &name = "");
+} // namespace Aidge
+
+namespace {
+template <>
+const char *const EnumStrings<Aidge::HeavisideAttr>::data[] = {"value"};
+}
+
+#endif /* AIDGE_CORE_OPERATOR_HEAVISIDE_H_ */