Add ai21 llm (#8233)

* Added AI21 Labs LLM

* added AI21 to __init__.py for llms

* Added tests for AI21 llm

* Added AI21 llm Example notebook

* Added Link to AI21 example notebook in docs

* bugfix: linting errors when ai21 is not installed

* fix lint

* wip

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Co-authored-by: Simon Suo <simonsdsuo@gmail.com>