Standardize the way of getting model weights from Trainer

Is your feature request related to a problem? Please describe.
Certain trainers, like GanTrainer use several models, while the standard SupervisedTrainer only uses one model.

Describe the solution you'd like
There should be a standardized API (e.g., get_weights/model_states) that returns all model parameters. This would be useful to allow integration with other components, like MonaiAlgo for federated learning.

Having a standardized API would allow MonaiAlgo to easily support bundles that use Trainers other than SupervisedTrainer, e.g. mednist_gan.

Describe alternatives you've considered
n/a

Additional context
n/a