[M0-Dev] Callbacks support

[pab1s]

I want to refactor the code to support callbacks. It is necessary to implement early stopping, so I decided that the best way to do it will be using callbacks. Moreover, I will change the way I log to do it using callbacks. My intention is to make the code cleaner and easier to understand.

With respect to early stopping, I will consider the following hyperparameters:

• monitor: to set if I want to monitor train or valid losses.
• patience: how many epochs does the model update the weights with no improvements.
• delta: min difference of losses between epochs that are considered an improvement.

Probably I will also add support to verbosity, so it is easier to track.

[pab1s]

Actually, this is basically finished. However, in a future it could be interesting to refactor schedulers or even metrics into callbacks. By now, I will leave it as it is.