[Naming] lm_labels -> labels ; masked_lm_labels -> masked_labels
This PR is a simple find and replace of the regex "lm_labels" into "labels" over all files.
This way we can use the Trainer class for all models. 

It does break backward compatibility a bit though, since some people will have to rename the `lm_labels` argument in their code. 


Yeah that's quite a big breaking change 🤣
we may want to be a little bit careful on backward compatibility<|||||>I think this would be a very welcome update. Maybe adding an alias to the method signature would be sufficient to maintain compatibility?

We could deprecate the old ones as well, with a warning and remove them in a future version.<|||||>Yes adding an alias would be good for me.

One possible option to deprecate progressively (that Keras use for instance) is to move the old parameters in `**kwargs` so they are not advocated anymore and add a deprecation warning later on.