Add Merge input_shape property

Author: fchollet

keras/layers/core.py

@@ -517,6 +517,10 @@ def __init__(self, layers, mode='sum', concat_axis=-1, dot_axes=-1):
                     self.constraints.append(c)
         super(Merge, self).__init__()
 
+    @property
+    def input_shape(self):
+        return [layer.input_shape for layer in self.layers]
+
     @property
     def output_shape(self):
         input_shapes = [layer.output_shape for layer in self.layers]