Allow choosing a model by name from CLI

[siemdejong]

Is your feature request related to a problem? Please describe.
The model has to be defined in a config file. It would be nice if --model CCMIL just works.

Describe the solution you'd like
Lightning CLI way of doing this

And include the defaults in the signature of CCMIL/VarMIL/ABMIL.

[siemdejong]

Might be more difficult, since trainer.max_epochs needs to be linked to model.T_max. Can this simply be done with argument linking?