Add UniPELT implementation

[calpt]

This PR adds the UniPELT adapter setup proposed in "UNIPELT: A Unified Framework for Parameter-Efficient Language Model Tuning" by Mao et al. (2022).

• Paper: https://aclanthology.org/2022.acl-long.433.pdf
• Reference implementation: https://github.com/morningmoni/UniPELT
• Implementation in adapter-transformers:
  • UniPELTConfig class that inherits from ConfigUnion
  • New config options use_gating for LoRA, prefix tuning and adapters to add gates introduced by UniPELT
  • New config option shared_gating for prefix tuning that determines whether all attention matrix prefixes share the same gate
  • New output_adapter_gating_scores parameter for model forward pass to return computed gating scores
• Documentation in adapter-transformers: https://github.com/calpt/adapter-transformers/blob/dev/unipelt/adapter_docs/overview.md#unipelt