callback

[emekaokoli19]

@ShreyaR sorry for taking a while, this is kind of what I imagine the callback would look like, also creating an abstract class with abstractmethod would mean the user would have to define all the methods in the subclass to be able to use callbacks which is unnecessary. I also have some questions about the arguments

e.g If a user wants a customized logic for preprocessing, does this mean the prepare function for preprocessing in run.py should be skipped? In that case, what arguments should the after_prepare function take, I assume it should be the return values of before_prepare

I would love to get your thoughts on this

[zsimjee]

Created a discussion here #612