Standardize the use of the model_name argument

[RobinPicard]

Addresses #1583

We standardize the behavior by including an optional model_name argument in the loading function of all models for which a model name can be provided (so all models except those of type SteerableModel and TGI).

In the text generation methods, we give precedence to the inference keyword argument for the model name, otherwise we fall back to the model_name value provided at initialization (if there's one)