Add Softmax support and throw exception if activation isn't found

Author: Hallupa

src/TensorFlowNET.Keras/Activations/Activations.Softmax.cs

@@ -0,0 +1,11 @@
+using System;
+using static Tensorflow.Binding;
+
+namespace Tensorflow.Keras
+{
+    public partial class Activations
+    {
+        public Activation Softmax = (features, name) 
+            => tf.Context.ExecuteOp("Softmax", name, new ExecuteOpArgs(features));
+    }
+}

src/TensorFlowNET.Keras/Layers/LayersApi.cs

@@ -1,4 +1,5 @@
-using Tensorflow.NumPy;
+using System;
+using Tensorflow.NumPy;
 using System.Collections.Generic;
 using Tensorflow.Keras.ArgsDefinition;
 using Tensorflow.Keras.Engine;
@@ -834,7 +835,8 @@ Activation GetActivationByName(string name)
                 "relu" => keras.activations.Relu,
                 "sigmoid" => keras.activations.Sigmoid,
                 "tanh" => keras.activations.Tanh,
-                _ => keras.activations.Linear
+                "softmax" => keras.activations.Softmax,
+                _ => throw new Exception($"Activation {name} not found")
             };
 
         /// <summary>