Change API for adversarial method

[tgnassou]

For now, the domain classifier is initialized in the DomainAwareModule. This leads to an unclear setup for new users because someone who wants to avoid adversarial can have to deal with domain classifiers.

The goal would be to put the domain classifier in the adversarial losses. The domain output will be computed in the loss, and only adversarial methods will deal with the domain classifiers.

[ambroiseodt]

@tgnassou do you have a detailed list of changes you want to implement there?