Fix a initiate chats (#1938)

* Fix async a_initiate_chats

* Fix type compatibility for python 3.8

* Use partial func to fix context error

* website/docs/tutorial/assets/conversable-agent.jpg: convert to Git LFS

* Update notebook examples

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Co-authored-by: Chi Wang <wang.chi@microsoft.com>
Co-authored-by: Davor Runje <davor@airt.ai>