Change default device selection

[chschroeder]

Feature description

TransformerBasedClassification and SetFitClassification should select a GPU if available and no device was set (device=None which is the default).

Motivation

Other libraries that small-text uses, such as transformers, sentence-transformers, or setfit check if a GPU device is available and use it by default.

We should adapt the same behavior, this makes it easier for new users and CPU-only usage can still be forced by setting the device parameter.

Additional comments

Thanks to @eisioriginal for reporting