Keras Multi-inputs AttributeError: 'NoneType' object has no attribute 'inbound_nodes'

keras.backend.repeat is a function, not a layer. Try using keras.layers.core.RepeatVector instead. It has the same functionality as the function.

emd_out_3d = RepeatVector(10)(emd_out)

Not just for that case, but in general case, if you like to add some function into your model whose has no equivalent layer implementation, you can make that function as Lambda layer.

for instance, I needed to add mean operator on axis=1 into my model. Here is the code as supposed my current tensor named xinput and output tensor is output the code should be as follows.

# suppose my tensor named xinput
meaner=Lambda(lambda x: K.mean(x, axis=1) )
agglayer = meaner(xinput)    
output = Dense(1, activation="linear", name="output_layer")(agglayer)

Instead of using Lambda function, but adding K.mean function directly, you will get the same error.