support the use of multiple hooks by checking for tuples form Composites' module_map and registering all of them (multiple Hook instances only overwrite the results of another)
add HookBase as another layer of abstraction, above both Hook (which hooks over one full module) and GradOutHook (which hooks only the output of a function)
add GradOutHook, which can modify the gradient at the output of a module
add MultiComposite as a parent to MixedComposite
add MergedComposite, inheriting from MultiComposite, which merges rules returned from composites for a multi-hook setting rather than using the first hook that applies
Todo: