Keras layer activations fix

Author: MPnoy

src/TensorFlowNET.Keras/Activations/Activations.Sigmoid.cs

@@ -17,7 +17,9 @@ public partial class Activations
                 return results[0];
             }
 
-            throw new NotImplementedException("");
+            var _op = tf.OpDefLib._apply_op_helper("Sigmoid", name: name, args: new { x = features });
+
+            return _op.output;
         };
     }
 }

src/TensorFlowNET.Keras/Activations/Activations.Tanh.cs

@@ -17,7 +17,9 @@ public partial class Activations
                 return results[0];
             }
 
-            throw new NotImplementedException("");
+            var _op = tf.OpDefLib._apply_op_helper("Tanh", name: name, args: new { x = features });
+
+            return _op.output;
         };
     }
 }