Fixed #398, Recusive defualt session factory

src/TensorFlowNET.Core/Keras/backend.cs

@@ -28,7 +28,7 @@ namespace Tensorflow.Keras
         //Func<Array, bool> py_any = any;
         //Func<double, double, double, IEnumerable<double>> py_slice = slice;
 
-        public static Session _SESSION = tf.defaultSession;
+        public static Session _SESSION = ops.get_default_session();
         public static Graph _GRAPH = null;
         public static Dictionary<Graph, GraphLearningPhase> _GRAPH_LEARNING_PHASES;
         //Dictionary<Graph, Dictionary<string, int>> PER_GRAPH_LAYER_NAME_UIDS;

src/TensorFlowNET.Core/tensorflow.cs

@@ -40,7 +40,7 @@ namespace Tensorflow
 
         public tensorflow()
         {
-            _defaultSessionFactory = new ThreadLocal<Session>(Session);
+            _defaultSessionFactory = new ThreadLocal<Session>(() => new Session());
         }
 
         public Session defaultSession => _defaultSessionFactory.Value;