support retrievaling assistant by name (#718)

* support assistant retrieval using name

* address comment

* Update autogen/agentchat/contrib/gpt_assistant_agent.py

Co-authored-by: Chi Wang <wang.chi@microsoft.com>

* make code more reading friendly

* ignore test error

* format code

* Update autogen/agentchat/contrib/gpt_assistant_agent.py

typo fix

* fix test case

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