Pjobic/refactoring/loss plugin

New plugin to dynamically use different loss functions.

How to use

in .yml, just put something like that:

loss_config: # concerns __init__
   loss_name: "default_loss_funcs" # the one already in use
   some_other_init_parameters: 0.0
loss_params: # concerns forward
   some_params_for_the_forward_pass: "blabla"

or

# nothing, By default, it uses `default_loss_funcs` like it is already doing in the master branch.

or

loss_config:
   loss_name: "scenario_wise_loss" # it will use it automatically

Need to TODO

• dynamically have these loss_params like it would be in the code. e.g.:

loss_params:
   learned_params: "self.learned_params" # automatically put self.learned_params from ModelTrainer.attr to loss_params dict

• get rid of errors_mean and errors_grouped, like adding a pluging for visualization/logging bcs it is specific to someone's task and should not be handled in training loop. (as you can see my scenario_wise_loss do not compute these errors and should not)

Any questions/remarks @jcury @embray ?