fixed Merge Layer functional API (#2460)

* fixed Merge Layer functional API

* moved test to layers/test_core

keras/engine/topology.py

@@ -1264,6 +1264,9 @@ def __call__(self, inputs, mask=None):
                                        node_indices, tensor_indices)
             self.built = True
             self.add_inbound_node(layers, node_indices, tensor_indices)
+
+            outputs = self.inbound_nodes[-1].output_tensors
+            return outputs[0] # merge only returns a single tensor
         else:
             return self.call(inputs, mask)
 

tests/keras/layers/test_core.py

@@ -14,7 +14,7 @@ def test_masking():
 
 
 def test_merge():
-    from keras.layers import Input, merge
+    from keras.layers import Input, merge, Merge
     from keras.models import Model
 
     # test modes: 'sum', 'mul', 'concat', 'ave', 'cos', 'dot'.
@@ -38,6 +38,16 @@ def test_merge():
         model = Model.from_config(config)
         model.compile('rmsprop', 'mse')
 
+        # test Merge (#2460)
+        merged = Merge(mode=mode)([input_a, input_b])
+        model = Model([input_a, input_b], merged)
+        model.compile('rmsprop', 'mse')
+
+        expected_output_shape = model.get_output_shape_for(input_shapes)
+        actual_output_shape = model.predict(inputs).shape
+        assert expected_output_shape == actual_output_shape
+
+
     # test lambda with output_shape lambda
     input_a = Input(shape=input_shapes[0][1:])
     input_b = Input(shape=input_shapes[1][1:])