How to train this model?

Hi, could someone tell me how to train A3C on the CARLA simulator?
I believe I should use A3CTrainer in a3c.py, but I cannot find where this class is used to train the model.
Also, I am not sure how the reward function is defined in A3C. Is it manually defined in somewhere in the codes or is it pre-defined by the CARLA simulator?

Thank you!