Extend benchmark_trainers

[accosmin]

• save the train & validation loss values/errors as csv
• plot & compare their evolution with the number of iterations & seconds
• extend the csv produced by trainer_state with an additional column as training time in seconds

[accosmin]

Also make the plotting scripts more user friendly (e.g. usage & input checks) and more modular/compact.

[accosmin]

Improve axis labels: always use epochs for Ox or seconds (if plotting by time), use loss or error for Oy.

[accosmin]

Thinner lines: it is very hard to zoom in and to distinguish the training loss from the validation loss.