[FEATURE] Make kerosene compatible with HyperOpt

[banctilrobitaille]

Is your feature request related to a problem? Please describe.
The train method does not return anything so it is currently not that friendly to use an Hyperparameters tuning lib such as HyperOpt : https://github.com/hyperopt/hyperopt

Describe the solution you'd like
The train method from Trainer would return the test monitors so its easy for the user to create an objective function compatible with hyperopt

Describe alternatives you've considered
Using the attribute on the trainer at the end of the training.

Will this change the current api? How?
no