thank you!
It just that I wanted to combine different combination of models. To do so I want to make a big model which will call m1(m2()) or m2(m1()) or m1(m1()) etc according to some trainable parameters. And i was wondering if the backward was not stuck to one model. But according to your answer the compose of model will make only graph so one backward. That s exactly what I need 
thank you again!