Avoid having two classes named Model

[brimoor]

It is confusing that we have two Model classes in ETA:

• eta.core.learning.Model: abstract base class for loading and performing inference with models
• eta.core.models.Model: Serializable class that describes a model, including its name, location of its model weights, and the default settings for using it

Perhaps we should rename eta.core.models.Model to eta.core.models.ModelDescription, to clarify that it describes a model and is not actually the model itself.