Fix the error of loading model saved before tf2.5.

src/TensorFlowNET.Core/APIs/tf.saved_model.cs

@@ -0,0 +1,20 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+using Tensorflow.Train;
+
+namespace Tensorflow
+{
+    public partial class tensorflow
+    {
+        public SavedModelAPI saved_model { get; } = new SavedModelAPI();
+    }
+
+    public class SavedModelAPI
+    {
+        public Trackable load(string export_dir, LoadOptions? options = null)
+        {
+            return Loader.load(export_dir, options);
+        }
+    }
+}

src/TensorFlowNET.Core/Graphs/FuncGraph.cs

@@ -8,6 +8,7 @@ using Tensorflow.Exceptions;
 using Tensorflow.Framework;
 using Tensorflow.Framework.Models;
 using Tensorflow.Functions;
+using Tensorflow.NumPy;
 using Tensorflow.Operations;
 using Tensorflow.Util;
 using static Tensorflow.Binding;
@@ -181,7 +182,7 @@ public class FuncGraph : Graph, IDisposable
     const int _EAGER_CONST_THRESHOLD = 128;
     public Tensor capture(Tensor tensor, string name = null, Shape shape = null)
     {
-        if(tensor is EagerTensor)
+        if(tensor is EagerTensor or NDArray)
         {
             if (name == null)
                 name = ops.uid().ToString();

src/TensorFlowNET.Core/Keras/Engine/IOptimizer.cs

@@ -10,4 +10,5 @@ public interface IOptimizer
     void apply_gradients(IEnumerable<(Tensor, IVariableV1)> grads_and_vars,
             string name = null,
             bool experimental_aggregate_gradients = true);
+    IVariableV1 add_slot(IVariableV1 var, string slot_name, IInitializer initializer = null);
 }

src/TensorFlowNET.Core/Operations/Operation.cs

@@ -216,10 +216,12 @@ namespace Tensorflow
         public virtual object get_attr(string name)
         {
             var buf = new Buffer();
-            c_api.TF_OperationGetAttrValueProto(_handle, name, buf, tf.Status);
-            tf.Status.Check(true);
+            Status status = new();
+            c_api.TF_OperationGetAttrValueProto(_handle, name, buf, status);
+            status.Check(true);
+            var tf_buffer = c_api.TF_GetBuffer(buf);
 
-            var x = AttrValue.Parser.ParseFrom(buf.ToArray());
+            var x = AttrValue.Parser.ParseFrom(tf_buffer.AsSpan<byte>());
 
             var oneof_value = x.ValueCase;
             if (oneof_value == AttrValue.ValueOneofCase.None)

src/TensorFlowNET.Core/Tensors/tensor_util.cs

@@ -64,36 +64,68 @@ namespace Tensorflow
             var num_elements = shape.size;
             var tensor_dtype = tensor.Dtype.as_tf_dtype();
 
+            T[] ExpandArrayToSize<T>(IList<T> src)
+            {
+                if(src.Count == 0)
+                {
+                    return new T[0];
+                }
+                var pad_count = num_elements - src.Count;
+                var pre = pad_count / 2;
+                var after = pad_count - pre;
+                var first_elem = src[0];
+                var last_elem = src[src.Count - 1];
+                T[] res = new T[num_elements];
+                for(long i = 0; i < num_elements; i++)
+                {
+                    if (i < pre) res[i] = first_elem;
+                    else if (i >= num_elements - after) res[i] = last_elem;
+                    else res[i] = src[(int)(i - pre)];
+                }
+                return res;
+            }
+
             if (shape.ndim > 0 && tensor.TensorContent.Length > 0)
             {
                 return np.frombuffer(tensor.TensorContent.ToByteArray(), shape, tensor_dtype);
             }
-            else if (tensor.Dtype == DataType.DtHalf || tensor.Dtype == DataType.DtBfloat16)
+            NDArray values;
+            if (tensor.Dtype == DataType.DtHalf || tensor.Dtype == DataType.DtBfloat16)
             {
-                return np.array(tensor.HalfVal.ToArray()).reshape(shape);
+                values = np.array(ExpandArrayToSize(tensor.HalfVal));
             }
             else if (tensor.Dtype == DataType.DtFloat)
             {
-                return np.array(tensor.FloatVal.ToArray()).reshape(shape);
+                values = np.array(ExpandArrayToSize(tensor.FloatVal));
             }
             else if (new DataType[] { DataType.DtInt32, DataType.DtUint8 }.Contains(tensor.Dtype))
             {
-                return np.array(tensor.IntVal.ToArray()).reshape(shape);
+                values = np.array(ExpandArrayToSize(tensor.IntVal));
             }
             else if (new DataType[] { DataType.DtInt64 }.Contains(tensor.Dtype))
             {
-                return np.array(tensor.Int64Val.ToArray()).reshape(shape);
+                values = np.array(ExpandArrayToSize(tensor.Int64Val));
             }
             else if (new DataType[] { DataType.DtUint64 }.Contains(tensor.Dtype))
             {
-                return np.array(tensor.Uint64Val.ToArray()).reshape(shape);
+                values = np.array(ExpandArrayToSize(tensor.Uint64Val));
             }
             else if (tensor.Dtype == DataType.DtBool)
             {
-                return np.array(tensor.BoolVal.ToArray()).reshape(shape);
+                values = np.array(ExpandArrayToSize(tensor.BoolVal));
+            }
+            else
+            {
+                throw new TypeError($"Unsupported tensor type: {tensor.Dtype}. See " +
+                    $"https://www.tensorflow.org/api_docs/python/tf/dtypes for supported TF dtypes.");
+            }
+
+            if(values.size == 0)
+            {
+                return np.zeros(shape, tensor_dtype);
             }
 
-            throw new NotImplementedException("MakeNdarray");
+            return values.reshape(shape);
         }
 
         private static readonly TF_DataType[] quantized_types = new TF_DataType[]

src/TensorFlowNET.Core/Trackables/TrackableConstant.cs

@@ -1,5 +1,6 @@
 using Google.Protobuf.Collections;
 using Tensorflow.Train;
+using static Tensorflow.Binding;
 
 namespace Tensorflow.Trackables;
 
@@ -11,12 +12,23 @@ public class TrackableConstant : Trackable
         _constant = constant;
     }
 
-    public static (Trackable, Action<object, object, object>) deserialize_from_proto(SavedObject object_proto,
+    public static (Tensor, Action<object, object, object>) deserialize_from_proto(SavedObject object_proto,
         Dictionary<string, MapField<string, AttrValue>> operation_attributes)
     {
         var tensor_proto = operation_attributes[object_proto.Constant.Operation]["value"].Tensor;
         var ndarray = tensor_util.MakeNdarray(tensor_proto);
-        var imported_constant = constant_op.constant(ndarray);
-        return (new TrackableConstant(imported_constant), null);
+        Tensor imported_constant;
+        if (tensor_proto.Dtype == DataType.DtString)
+        {
+            imported_constant = tf_with(ops.device("CPU"), _ =>
+            {
+                return constant_op.constant(ndarray);
+            });
+        }
+        else
+        {
+            imported_constant = constant_op.constant(ndarray);
+        }
+        return (imported_constant, null);
     }
 }

src/TensorFlowNET.Core/Training/Saving/SavedModel/RevivedTypes.cs

@@ -46,4 +46,9 @@ public class RevivedTypes
             return (null, null);
         }
     }
+
+    public static void RegisterRevivedTypeCreator(string identifier, ITrackableWrapper obj)
+    {
+        _registered_revived_creator[identifier] = obj;
+    }
 }

src/TensorFlowNET.Core/Training/Saving/SavedModel/SaveableView.cs

@@ -137,7 +137,7 @@ public class SaveableView
     /// </summary>
     public List<int> dependency_sorted_node_ids()
     {
-        Dictionary<int, IEnumerable<int>> dependency_map = new();
+        Dictionary<int, List<int>> dependency_map = new();
         foreach (var node in _nodes)
         {
             var node_id = _node_ids[node];

src/TensorFlowNET.Core/Training/Saving/SavedModel/function_deserialization.cs

@@ -116,17 +116,23 @@ namespace Tensorflow.Training.Saving.SavedModel
             }
 
             Dictionary<string, ConcreteFunction> loaded_gradients = new();
-            foreach (var fdef in _sort_function_defs(library, function_deps))
+            // Debug(Rinne)
+            var temp = _sort_function_defs(library, function_deps);
+            int i = 0;
+            foreach (var fdef in temp)
             {
+                i++;
                 var orig_name = _fix_fdef_in_place(fdef, functions, load_shared_name_suffix, new_gradient_op_types);
 
                 object structured_input_signature = null;
                 object structured_outputs = null;
                 if (saved_object_graph is not null && saved_object_graph.ConcreteFunctions.ContainsKey(orig_name))
                 {
-                    var proto = saved_object_graph.ConcreteFunctions[orig_name];
-                    structured_input_signature = nested_structure_coder.decode_proto(proto.CanonicalizedInputSignature);
-                    structured_outputs = nested_structure_coder.decode_proto(proto.OutputSignature);
+                    // TODO(Rinne): deal with structured_input_signature and structured_outputs.
+
+                    //var proto = saved_object_graph.ConcreteFunctions[orig_name];
+                    //structured_input_signature = nested_structure_coder.decode_proto(proto.CanonicalizedInputSignature);
+                    //structured_outputs = nested_structure_coder.decode_proto(proto.OutputSignature);
                 }
 
                 graph.as_default();
@@ -234,27 +240,41 @@ namespace Tensorflow.Training.Saving.SavedModel
 
         private static void _restore_gradient_functions(FuncGraph func_graph, Dictionary<string, ConcreteFunction> renamed_functions, Dictionary<string, ConcreteFunction> loaded_gradients)
         {
-            foreach(var op in func_graph.get_operations())
+            if(loaded_gradients is null || loaded_gradients.Count == 0)
             {
-                if(op.op.type == "StatefulPartitionedCall" || op.op.type == "PartitionedCall")
-                {
-                    var function = renamed_functions[op.op.node_def.Attr["f"].Func.Name];
-                    op.op._gradient_function = function._get_gradient_function();
-                }
-                string gradient_op_type = null;
-                try
-                {
-                    gradient_op_type = op.op.get_attr("_gradient_op_type") as string;
-                }
-                catch(InvalidArgumentError)
+                foreach (var op in func_graph.get_operations())
                 {
-                    continue;
+                    if (op.op.type == "StatefulPartitionedCall" || op.op.type == "PartitionedCall")
+                    {
+                        var function = renamed_functions[op.op.node_def.Attr["f"].Func.Name];
+                        op.op._gradient_function = function._get_gradient_function();
+                    }
                 }
-                if (loaded_gradients.ContainsKey(gradient_op_type))
+            }
+            else
+            {
+                foreach (var op in func_graph.get_operations())
                 {
-                    var grad_fn = loaded_gradients[gradient_op_type];
-                    grad_fn.NumPositionArgs = op.op.inputs.Length;
-                    grad_fn.ArgKeywords = op.op.inputs._inputs.Select(x => x.name);
+                    if (op.op.type == "StatefulPartitionedCall" || op.op.type == "PartitionedCall")
+                    {
+                        var function = renamed_functions[op.op.node_def.Attr["f"].Func.Name];
+                        op.op._gradient_function = function._get_gradient_function();
+                    }
+                    string gradient_op_type = null;
+                    try
+                    {
+                        gradient_op_type = op.op.get_attr("_gradient_op_type") as string;
+                    }
+                    catch (InvalidArgumentError)
+                    {
+                        continue;
+                    }
+                    if (loaded_gradients.ContainsKey(gradient_op_type))
+                    {
+                        var grad_fn = loaded_gradients[gradient_op_type];
+                        grad_fn.NumPositionArgs = op.op.inputs.Length;
+                        grad_fn.ArgKeywords = op.op.inputs._inputs.Select(x => x.name);
+                    }
                 }
             }
         }

src/TensorFlowNET.Core/Training/Saving/SavedModel/loader.cs

@@ -15,6 +15,7 @@ using Tensorflow.Functions;
 using Tensorflow.Training.Saving.SavedModel;
 using Tensorflow.Trackables;
 using OneOf;
+using Tensorflow.Keras.Engine;
 
 namespace Tensorflow
 {
@@ -34,7 +35,7 @@ namespace Tensorflow
         private List<int>? _filtered_nodes;
         private List<int> _ordered_node_ids;
         private Dictionary<int, (Trackable, Action<object, object, object>)> _loaded_nodes;
-        private List<Trackable> _nodes;
+        private List<object> _nodes;
         private Dictionary<int, Action<object, object, object>> _node_setters;
         private Dictionary<string, ConcreteFunction> _concrete_functions;
         private HashSet<string> _restored_concrete_functions;
@@ -213,7 +214,13 @@ namespace Tensorflow
                     continue;
                 }
                 var proto = _proto.Nodes[node_id];
-                foreach(var dep in _get_node_dependencies(proto).Values.Distinct())
+                if(node_id == 10522)
+                {
+                    // Debug(Rinne)
+                    Console.WriteLine();
+                }
+                var temp = _get_node_dependencies(proto);
+                foreach (var dep in _get_node_dependencies(proto).Values.Distinct())
                 {
                     deps.Add(dep);
                     if(_filtered_nodes is not null && !_filtered_nodes.Contains(dep))
@@ -232,7 +239,7 @@ namespace Tensorflow
                     // The optimizer and original variable must be created before the slot
                     // variable, since the slot variable is generated using the Optimizer's
                     // add_slot API.
-                    var slot_deps = dependency_map[slot_variable_node_id];
+                    var slot_deps = dependency_map.SetDefault(slot_variable_node_id, new List<int>());
                     slot_deps.Add(node_id);
                     slot_deps.Add(slot_variable_proto.OriginalVariableNodeId);
 
@@ -245,7 +252,12 @@ namespace Tensorflow
             }
             try
             {
-                return TrackableUtils.order_by_dependency(dependency_map.ToDictionary(x => x.Key, x => x.Value as IEnumerable<int>));
+                int total = 0;
+                foreach(var v in dependency_map.Values)
+                {
+                    total += v.Count;
+                }
+                return TrackableUtils.order_by_dependency(dependency_map);
             }
             catch (TrackableUtils.CyclicDependencyError ex)
             {
@@ -339,9 +351,20 @@ namespace Tensorflow
                         var saveable_object_proto = item.Value;
                         var save_fn_id = saveable_object_proto.SaveFunction;
                         var restore_fn_id = saveable_object_proto.RestoreFunction;
-                        saveable_fn_by_name[name] = (get(save_fn_id), get(restore_fn_id));
+                        saveable_fn_by_name[name] = ((Trackable)get(save_fn_id), (Trackable)get(restore_fn_id));
+                    }
+                    var saveable_objects = saveable_object_util.recreate_saveable_objects(saveable_fn_by_name, null);
+                    if (saveable_objects is not null && saveable_objects.Count > 0)
+                    {
+                        if(node is Trackable trackable)
+                        {
+                            trackable.SelfSaveableObjectFactories = saveable_objects;
+                        }
+                        else
+                        {
+                            throw new TypeError();
+                        }
                     }
-                    node.SelfSaveableObjectFactories = saveable_object_util.recreate_saveable_objects(saveable_fn_by_name, null);
                 }
             }
         }
@@ -379,12 +402,12 @@ namespace Tensorflow
                 {
                     // Use the public Optimizer interface when creating slot variables.
                     var (optimizer_node_id, slot_variable_proto) = slot_variable_node_ids[node_id];
-                    var optimizer_object = nodes[optimizer_node_id];
+                    var optimizer_object = nodes[optimizer_node_id] as IOptimizer;
                     var optimizer_variable = nodes[slot_variable_proto.OriginalVariableNodeId];
 
-                    // TODO(Rinne): implement it.
-                    throw new NotImplementedException("The model loading of SavedModel still has some incompleted part." +
-                        " Please submit an issue to https://github.com/SciSharp/TensorFlow.NET/issues.");
+                    var slot_variable = optimizer_object.add_slot(optimizer_variable as IVariableV1, slot_variable_proto.SlotName);
+                    nodes[slot_variable_proto.SlotVariableNodeId] = slot_variable as Trackable;
+                    node_setters[slot_variable_proto.SlotVariableNodeId] = setattr;
                 }
                 else
                 {
@@ -398,7 +421,7 @@ namespace Tensorflow
             {
                 nodes[0] = _recreate_base_user_object().Item1;
             }
-            _nodes = new List<Trackable>();
+            _nodes = new List<object>();
             for(int i = 0; i < _proto.Nodes.Count; i++)
             {
                 _nodes.Add(nodes[i]);
@@ -412,7 +435,7 @@ namespace Tensorflow
         private void _restore_checkpoint()
         {
             var variables_path = SavedModelUtils.get_variables_path(_export_dir);
-            var saver = new TrackableSaver(new ObjectGraphView(get(0)));
+            var saver = new TrackableSaver(new ObjectGraphView((Trackable)get(0)));
             tf_with(ops.device("CPU"), _ =>
             {
                 saver.FilePrefixPlaceHolder = constant_op.constant(variables_path);
@@ -467,7 +490,7 @@ namespace Tensorflow
             }
         }
 
-        private void _setup_function_captures(string concrete_function_name, IDictionary<OneOf<string, int>, Trackable> nodes)
+        private void _setup_function_captures(string concrete_function_name, IDictionary<OneOf<string, int>, object> nodes)
         {
             if (_restored_concrete_functions.Contains(concrete_function_name))
             {
@@ -485,12 +508,12 @@ namespace Tensorflow
            // TODO: implement it with concrete functions.
         }
 
-        public Trackable get(int node_id)
+        public object get(int node_id)
         {
             return _nodes[node_id];
         }
 
-        public Trackable get(string node_id)
+        public object get(string node_id)
         {
             return get(_node_path_to_id[node_id]);
         }
@@ -512,9 +535,9 @@ namespace Tensorflow
             }
         }
 
-        private (Dictionary<int, Trackable>, Dictionary<int, Action<object, object, object>>) _initialize_loaded_nodes()
+        private (Dictionary<int, object>, Dictionary<int, Action<object, object, object>>) _initialize_loaded_nodes()
         {
-            Dictionary<int, Trackable> nodes = new();
+            Dictionary<int, object> nodes = new();
             Dictionary<int, Action<object, object, object>> node_setters = new();
             foreach(var item in _loaded_nodes)
             {
@@ -534,10 +557,10 @@ namespace Tensorflow
             }
         }
 
-        private (Trackable, Action<object, object, object>) _recreate(SavedObject proto, int node_id, IDictionary<int, Trackable> nodes)
+        private (object, Action<object, object, object>) _recreate(SavedObject proto, int node_id, IDictionary<int, object> nodes)
         {
             // skip the registered classes.
-            Dictionary<OneOf<string, int>, Trackable> dependencies = new();
+            Dictionary<OneOf<string, int>, object> dependencies = new();
             foreach(var item in _get_node_dependencies(proto))
             {
                 dependencies[item.Key] = nodes[item.Value];
@@ -558,7 +581,7 @@ namespace Tensorflow
         /// <param name="proto"></param>
         /// <param name="node_id"></param>
         /// <param name="dependencies"></param>
-        private (Trackable, Action<object, object, object>) _recreate_default(SavedObject proto, int node_id, IDictionary<OneOf<string, int>, Trackable> dependencies)
+        private (Trackable, Action<object, object, object>) _recreate_default(SavedObject proto, int node_id, IDictionary<OneOf<string, int>, object> dependencies)
         {
             return proto.KindCase switch
             {
@@ -626,7 +649,7 @@ namespace Tensorflow
         }
 
         private (Function, Action<object, object, object>) _recreate_function(SavedFunction proto,
-            IDictionary<OneOf<string, int>, Trackable> dependencies)
+            IDictionary<OneOf<string, int>, object> dependencies)
         {
             var fn = function_deserialization.recreate_function(proto, _concrete_functions);
             foreach (var name in proto.ConcreteFunctions)
@@ -637,7 +660,7 @@ namespace Tensorflow
         }
 
         private (ConcreteFunction, Action<object, object, object>) _recreate_bare_concrete_function(SavedBareConcreteFunction proto,
-            IDictionary<OneOf<string, int>, Trackable> dependencies)
+            IDictionary<OneOf<string, int>, object> dependencies)
         {
             var fn = function_deserialization.setup_bare_concrete_function(proto, _concrete_functions);
             _setup_function_captures(proto.ConcreteFunctionName, dependencies);

src/TensorFlowNET.Core/Training/Saving/SavedModel/loader.static.cs

@@ -78,7 +78,7 @@ namespace Tensorflow
                 tf_with(ops.init_scope(), x =>
                 {
                     loader = new Loader(object_graph_proto, saved_model_proto, export_dir, ckpt_options, options, filters);
-                    root = loader.get(0);
+                    root = (Trackable)loader.get(0);
                     // skip the assignment of `graph_debug_info`.
                 });
                 // skip the assignment of `tensorflow_version`
@@ -99,7 +99,7 @@ namespace Tensorflow
             }
             if(filters != null && filters.Count > 0)
             {
-                return filters.Keys.ToDictionary(x => x, x => loader.get(x));
+                return filters.Keys.ToDictionary(x => x, x => (Trackable)loader.get(x));
             }
             else
             {

src/TensorFlowNET.Core/Training/TrackableUtils.cs

@@ -52,7 +52,7 @@ public static class TrackableUtils
     /// </summary>
     /// <param name="dependency_map"></param>
     /// <exception cref="ValueError"></exception>
-    public static List<int> order_by_dependency(IDictionary<int, IEnumerable<int>> dependency_map)
+    public static List<int> order_by_dependency(IDictionary<int, List<int>> dependency_map)
     {
         Dictionary<int, HashSet<int>> reverse_dependency_map = new();
         foreach (var pair in dependency_map)
@@ -102,7 +102,7 @@ public static class TrackableUtils
                 edges.Remove(x);
                 if (edges.Count == 0)
                 {
-                    to_visit.Enqueue(dep);
+                     to_visit.Enqueue(dep);
                     if (!reverse_dependency_map.Remove(dep))
                     {
                         throw new KeyError($"Cannot find the key {dep} in reverse_dependency_map");

src/TensorFlowNET.Core/Variables/BaseResourceVariable.cs

@@ -333,5 +333,23 @@ namespace Tensorflow
             });
             return array_ops.identity(value);
         }
+
+        //public static Tensor operator +(BaseResourceVariable x, int y) => x.value() + y;
+        //public static Tensor operator +(BaseResourceVariable x, float y) => x.value() + y;
+        //public static Tensor operator +(BaseResourceVariable x, double y) => x.value() + y;
+        //public static Tensor operator +(BaseResourceVariable x, BaseResourceVariable y) => x.value() + y.value();
+        //public static Tensor operator -(BaseResourceVariable x, int y) => x.value() - y;
+        //public static Tensor operator -(BaseResourceVariable x, float y) => x.value() - y;
+        //public static Tensor operator -(BaseResourceVariable x, double y) => x.value() - y;
+        //public static Tensor operator -(BaseResourceVariable x, Tensor y) => x.value() - y;
+        //public static Tensor operator -(BaseResourceVariable x, BaseResourceVariable y) => x.value() - y.value();
+
+        //public static Tensor operator *(BaseResourceVariable x, BaseResourceVariable y) => x.value() * y.value();
+        //public static Tensor operator *(BaseResourceVariable x, Tensor y) => x.value() * y;
+        //public static Tensor operator *(BaseResourceVariable x, NDArray y) => x.value() * y;
+
+        //public static Tensor operator <(BaseResourceVariable x, Tensor y) => x.value() < y;
+
+        //public static Tensor operator >(BaseResourceVariable x, Tensor y) => x.value() > y;
     }
 }

src/TensorFlowNET.Core/Variables/ResourceVariable.Operators.cs

@@ -1,19 +1,6 @@
-/*****************************************************************************
-   Copyright 2018 The TensorFlow.NET Authors. All Rights Reserved.
-
-   Licensed under the Apache License, Version 2.0 (the "License");
-   you may not use this file except in compliance with the License.
-   You may obtain a copy of the License at
-
-       http://www.apache.org/licenses/LICENSE-2.0
-
-   Unless required by applicable law or agreed to in writing, software
-   distributed under the License is distributed on an "AS IS" BASIS,
-   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-   See the License for the specific language governing permissions and
-   limitations under the License.
-******************************************************************************/
-
+using System;
+using System.Collections.Generic;
+using System.Text;
 using Tensorflow.NumPy;
 
 namespace Tensorflow

src/TensorFlowNET.Keras/BackendImpl.cs

@@ -169,6 +169,12 @@ namespace Tensorflow.Keras
             _GRAPH_LEARNING_PHASES[tf.get_default_graph()] = (GraphLearningPhase)((value) ? 1 : 0);
         }
 
+        public void set_value(IVariableV1 x, object value)
+        {
+            // TODO(Rinne): check the implementation.
+            x.assign(value);
+        }
+
         public void batch_set_value(List<(IVariableV1, NDArray)> tuples)
         {
             if (ops.executing_eagerly_outside_functions())

src/TensorFlowNET.Keras/KerasInterface.cs

@@ -36,6 +36,11 @@ namespace Tensorflow.Keras
             }
         }
 
+        static KerasInterface()
+        {
+            RevivedTypes.RegisterRevivedTypeCreator("optimizer", new RestoredOptimizer());
+        }
+
         public KerasDataset datasets { get; } = new KerasDataset();
         public IInitializersApi initializers { get; } = new InitializersApi();
         public Regularizers regularizers { get; } = new Regularizers();

src/TensorFlowNET.Keras/Optimizers/OptimizerV2.cs

@@ -14,11 +14,11 @@ namespace Tensorflow.Keras.Optimizers
         protected bool _hypers_created;
         protected virtual string _name { get; }
 
-        IVariableV1 _iterations;
+        protected IVariableV1 _iterations;
         protected ResourceVariable iterations => _iterations as ResourceVariable;
         List<IVariableV1> _weights;
-        Dictionary<string, float> _hyper;
-        Dictionary<string, IVariableV1> _hyper_variables;
+        protected Dictionary<string, float> _hyper;
+        protected Dictionary<string, IVariableV1> _hyper_variables;
         protected bool _momentum;
         protected float _initial_decay = 0.0f;
         protected bool _use_locking = true;
@@ -224,7 +224,7 @@ namespace Tensorflow.Keras.Optimizers
             }
         }
 
-        protected IVariableV1 add_slot(IVariableV1 var, string slot_name, IInitializer initializer = null)
+        public IVariableV1 add_slot(IVariableV1 var, string slot_name, IInitializer initializer = null)
         {
             if (initializer == null)
                 initializer = tf.zeros_initializer;

src/TensorFlowNET.Keras/Optimizers/RestoredOptimizer.cs

@@ -0,0 +1,63 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+using Tensorflow.Keras.Saving;
+using Tensorflow.Train;
+using Tensorflow.Training;
+
+namespace Tensorflow.Keras.Optimizers
+{
+    public class RestoredOptimizer: OptimizerV2, ITrackableWrapper, IKerasConfig
+    {
+        public String Identifier { get; } = "optimizer";
+        public int Version { get; } = 2;
+        public int MinConsumerVersion { get; } = 1;
+        public int MinProducerVersion { get; } = 1;
+        public RestoredOptimizer(): base(new ArgsDefinition.OptimizerV2Args() { Name = "RestoredOptimizer" })
+        {
+            _hypers_created = true;
+        }
+
+        public IKerasConfig get_config()
+        {
+            throw new NotImplementedException("Restoring functional Optimizers from SavedModels is not currently " +
+                "supported. Please file a feature request if this limitation bothers you.");
+        }
+
+        public void SetValue(object name, object value)
+        {
+            if(name is not String str)
+            {
+                throw new TypeError($"The name of value to set must be string, but got {name.GetType()}");
+            }
+            if(value is Trackable trackable)
+            {
+                _track_trackable(trackable, str, overwrite: true);
+            }
+            if(value is IVariableV1 resource_variable)
+            {
+                if (!_hyper_variables.ContainsKey(str))
+                {
+                    _hyper_variables[str] = resource_variable;
+                }
+                else
+                {
+                    keras.backend.set_value(resource_variable, value);
+                }
+            }
+            else if (value is float f)
+            {
+                _hyper[str] = f;
+            }
+            else
+            {
+                throw new NotImplementedException();
+            }
+        }
+        
+        public Trackable FromProto(SavedUserObject proto)
+        {
+            return new RestoredOptimizer();
+        }
+    }
+}

test/TensorFlowNET.Keras.UnitTest/SaveModel/SequentialModelLoad.cs

@@ -2,6 +2,7 @@
 using System;
 using System.Linq;
 using Tensorflow;
+using Tensorflow.Keras.Engine;
 using Tensorflow.Keras.Optimizers;
 using Tensorflow.Keras.UnitTest.Helpers;
 using Tensorflow.NumPy;
@@ -103,4 +104,13 @@ public class SequentialModelLoad
 
         classify_model.fit(x, y, batch_size: 4);
     }
+
+    [Ignore]
+    [TestMethod]
+    public void TestModelBeforeTF2_5()
+    {
+        var a = keras.layers;
+        var model = tf.saved_model.load(@"D:\development\temp\saved_model") as Model;
+        model.summary();
+    }
 }