[example/documentation] Experimenting with new loss functions

[harold]

A machine learner working with cortex reads about a loss function in a paper and wants to experiment with something similar.

A document and/or example explaining how to do this with cortex would be super-useful.

For example, this loss function:

From page 4 of the yolo paper. What is the correct way to do something like this in cortex?

That is just a single example, feel free to add others here.