Fix input dtype for MapDataset. #666

Author: Oceania2018

src/TensorFlowNET.Core/Data/MapDataset.cs

@@ -1,5 +1,6 @@
 using System;
 using Tensorflow.Functions;
+using static Tensorflow.Binding;
 
 namespace Tensorflow
 {
@@ -14,7 +15,12 @@ public MapDataset(IDatasetV2 input_dataset,
             bool preserve_cardinality = false,
             bool use_legacy_function = false) : base(input_dataset)
         {
-            var func = new ConcreteFunction(map_func, input_dataset.element_spec[0].dtype);
+            var func = new ConcreteFunction($"autograph_{map_func.Method.Name}");
+            var input = tf.placeholder(input_dataset.element_spec[0].dtype, name: "input");
+            var output = map_func(input);
+            func.ToGraph(input, output);
+
+            structure = func.OutputStructure;
 
             variant_tensor = ops.map_dataset(input_dataset.variant_tensor,
                 func,

src/TensorFlowNET.Core/Functions/ConcreteFunction.cs

@@ -109,6 +109,8 @@ public void ToGraph(Tensors inputs, Tensors outputs)
                 inputs,
                 outputs,
                 null);
+
+            OutputStructure = outputs.Select(x => x.ToTensorSpec()).ToArray();
         }
 
         public Tensors Invoke(Tensors inputs)