Add option to skip 'prepare_for_inference' to enable model training

🚀 Feature Request

I think it should be possible to pass 'prepare_for_inference=False' into the 'from_pretrained' method from classes that inherit from 'GeneratorHubInterface'.

Motivation

It is currently not possible to turn the calling of the 'prepare_for_inference' off. It optimizes inference, but it prohibits training. I think it would be nice to be able to fine-tune these models.

Pitch

The from_pretrained methods from the affected classes already have kwargs passed to them. These kwargs could optionally contain the argument 'prepare_for_inference=False/True'. The kwargs can be added to the constructor of the affected classes and passed down to the base class constructor of GeneratorHubInterface. There, the calling of the 'prepare_for_inference' for each of the models can be conditionally turned off, if the argument 'prepare_for_inference' is False. The argument 'prepare_for_inference' should be retrieved from kwargs with 'kwargs.get("prepare_for_inference", True)' to not change the now default behaviour.

I already implemented the required changes in a fork.