Convert ChatCompletionMessage to Dict after completion (#791)

* update

* update

* update signature

* update

* update

* fix test funccall groupchat

* reverse change

* update

* update

* update

* update

* update

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Co-authored-by: Qingyun Wu <qingyun.wu@psu.edu>
Co-authored-by: Chi Wang <wang.chi@microsoft.com>