Add a an extras/contrib package

Some models deviate so much from standard transformer encoder/decoders (e.g. DeBERTa and Falcon old architecture) that we probably should not support them in mainline Curated Transformers to avoid cluttering the abstractions too much (it's *curated* after all). Though it would still be nice to provide official support for these models. Consider an add-on package for these models.

Probably requires #346.