If I understand your scenario correctly you are creating your own child class, using the Trainer class as parent class, is that right? If so, you should be able to add any arguments you want to the child class, since it is yours to amend, no?
Just like in this example: How do I add arguments to a subclass in Python 3 - Stack Overflow 4
Let me know if this is helpful at all or if I complete misunderstood your question 
Cheers
Heiko