AFT-702: Add heterogeneous input support for GN2

Idea:
Create an abstract class for a constituent loader with the getData function, which returns a vector of values. Each loader (e.g., TracksLoader, IParticlesLoader, ...) is inherited from it. Then, we have a vector of pointers and sequentially apply all of them and feed output to GNN. Relates to AFT-702

TO-DOs:

• Add abstract class for constituent loading
• Add tracksLoader class
• Add tracksLoader configuration
• Add IParticleLoader class
• Clean code

Tagging @dguest @svanstro @ioleksiy

PhysicsAnalysis/JetTagging/FlavorTagDiscriminants/FlavorTagDiscriminants/ConstituentsLoader.h

+/*
+  Copyright (C) 2002-2024 CERN for the benefit of the ATLAS collaboration
+
+  This is a virtual class to represent loader of any type of constituents.
+  It defines the interface for loading constituents from a jet 
+  and extracting their features for the NN evaluation.
+*/
+
+#ifndef CONTITUENTS_LOADER_H
+#define CONTITUENTS_LOADER_H
+
+// local includes
+#include "FlavorTagDiscriminants/FlipTagEnums.h"
+#include "FlavorTagDiscriminants/AssociationEnums.h"
+#include "FlavorTagDiscriminants/OnnxUtil.h"
+#include "FlavorTagDiscriminants/FTagDataDependencyNames.h"
+#include "FlavorTagDiscriminants/StringUtils.h"
+
+// EDM includes
+#include "xAODJet/Jet.h"
+#include "xAODBTagging/BTagging.h"
+
+// STL includes
+#include <string>
+#include <vector>
+
+namespace FlavorTagDiscriminants {
+
+    enum class ConstituentsEDMType {CHAR, UCHAR, INT, FLOAT, DOUBLE, CUSTOM_GETTER};
+    enum class ConstituentsSortOrder {
+        ABS_D0_SIGNIFICANCE_DESCENDING,
+        D0_SIGNIFICANCE_DESCENDING,
+        PT_DESCENDING,
+        ABS_D0_DESCENDING
+    };
+    enum class ConstituentsSelection {
+        ALL,
+        IP3D_2018,
+        DIPS_TIGHT_UPGRADE,
+        DIPS_LOOSE_UPGRADE,
+        DIPS_LOOSE_202102,
+        LOOSE_202102_NOIP,
+        R22_DEFAULT,
+        R22_LOOSE
+    };
+    enum class ConstituentsType {
+        IPARTICLE,
+        TRACK
+    };
+
+    struct InputVariableConfig {
+        std::string name;
+        ConstituentsEDMType type;
+        bool flip_sign;
+    };
+
+    struct ConstituentsInputConfig {
+        std::string name;
+        std::string output_name;
+        ConstituentsType type;
+        ConstituentsSortOrder order;
+        ConstituentsSelection selection;
+        std::vector<InputVariableConfig> inputs;
+    };
+
+    ConstituentsInputConfig createConstituentsLoaderConfig(
+      std::string name,
+      std::vector<std::string> input_variables,
+      FlipTagConfig flip_config
+    );
+
+    // Virtual class to represent loader of any type of constituents
+    class IConstituentsLoader {
+        public:
+            IConstituentsLoader(ConstituentsInputConfig cfg) {
+              m_config = cfg;
+            };
+            virtual ~IConstituentsLoader() = default;
+            virtual std::tuple<std::string, input_pair, std::vector<const xAOD::IParticle*>> getData(
+                const xAOD::Jet& jet, 
+                [[maybe_unused]] const SG::AuxElement& btag) const = 0;
+            virtual FTagDataDependencyNames getDependencies() const = 0;
+            virtual std::set<std::string> getUsedRemap() const = 0;
+            virtual std::string getName() const = 0;
+            virtual ConstituentsType getType() const = 0;
+
+        protected:
+            FTagDataDependencyNames m_deps;
+            ConstituentsInputConfig m_config;
+            std::set<std::string> m_used_remap;
+            std::string m_name;
+    };
+}
+
+
+#endif
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