Based on transformers v4.5.1

All major new features & changes are described at https://docs.adapterhub.ml/v2_transition.

• all changes merged via #105

Additional changes & Fixes

• Support loading adapters with load_best_model_at_end in Trainer (@calpt via #122)
• Add setter for active_adapters property (@calpt via #132)
• New notebooks for NER, text generation & AdapterDrop (@hSterz via #135)
• Enable trainer to load adapters from checkpoints (@hSterz via #138)
• Update & clean up example scripts (@hSterz via #154 & @calpt via #141, #155)
• Add unfreeze_adapters param to train_fusion() (@calpt via #156)
• Ensure eval/ train mode is correct for AdapterFusion (@calpt via #157)