Review optim module

[thibaultdvx]

This PR is a review of the optim module created in #643 .

More precisely, I created one config class for each supported optimizer and LR scheduler, whereas before there was a single config class for all the optimizers and one for all the schedulers. This enables to create create_optimizer_config and create_lr_scheduler_config functions that are similar to create_network_config (see #647), create_metric_config (see #654) and create_loss_config (see #655) and therefore have a consistent behavior across all modules.

I also put all the Enum class in specific files.

[camillebrianceau]

LGTM !