Rewrite metric and loss utils

[simi2525]

Rewrite the utils to dynamically create the dictionary with the constructor, display name, and higher_is_better variables. Add the necessary variables in the constructors of those classes, and write a function that takes all the classes in the file and adds them to the dictionary.

Bonus points:
Maybe rethink the metric class to allow to accumulate the inputs of calculations and aggregate them only once per epoch.