Make xTuring config file optional or let users pass model config to BaseModel loading

### Description
BaseModel load does not allow for loading of weights from non-xTuring hub based models. We should either make it optional or let users provide more details of these models using kwargs.

This will also help with #132.

https://github.com/stochasticai/xturing/blob/668f61be7d45041f65a8dbea1b5bbf46604f0203/src/xturing/models/base.py#L18