Review of loss module

[thibaultdvx]

This PR is a review of the loss function module created in #640.

More precisely, I created one config class for each supported loss function, whereas before there was a single config class for all the losses. This enables to create a create_loss_config function that is similar to create_network_config (see #647) and create_metric_config (see #654), and therefore have a consistent behavior across all modules.

I also put all the Enum class in a specific file.

[camillebrianceau]

LGTM !