How to design a network that's able to train multiple learning rate?

Hi everyone.

When I use pre-defined model of fastai like resnet34, … I can run multiple learning rate training.
But when I self-define a deep network, it is just able to run normal training(constant learning rate or SGD with restarts), not able to run multiple learning rate.
It returns error:

According to my study, the pre-defined networks are constructed by 3 blocks of layers, so they can be trained by 3 individual learning rates for 3 individual blocks.
So I want to ask, how to code a network into blocks to run multiple learning rate training?