Recover `overfit` option for partitions

Created by: scarlehoff

At some point (don't know when) the documented option to overfit a fold was removed. This just puts it back in.

This is something that would only be using for debugging or when we want a student to do something very specific -like in this case- so if someone has a quick look (only one line!) I would appreciate it.

Assigning @RoyStegeman since he's the person who has the most experience with hyperopt (and so if this change is to break something, the person who most likely will realise it!)