Based on transformers v4.12.5

New

• Allow adding, loading & training of model embeddings (@hSterz via #245). See https://docs.adapterhub.ml/embeddings.html.

Changed

• Unify built-in & custom head implementation (@hSterz via #252)
• Upgrade of underlying transformers version (@calpt via #255)

Fixed

• Fix documentation and consistency issues for AdapterFusion methods (@calpt via #259)
• Fix serialization/ deserialization issues with custom adapter config classes (@calpt via #253)