Add agentchat_oai_assistant_groupchat.ipynb

[LeoLjl]

Why are these changes needed?

Add agentchat_oai_assistant_groupchat.ipynb

Related issue number

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[sonichi]

It's an interesting example. Thanks.

[codecov-commenter]

Codecov Report

Merging #656 (67c43a3) into main (6d621a9) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #656   +/-   ##
=======================================
  Coverage   28.70%   28.70%           
=======================================
  Files          27       27           
  Lines        3383     3383           
  Branches      760      760           
=======================================
  Hits          971      971           
  Misses       2341     2341           
  Partials       71       71           

Flag	Coverage Δ
unittests	28.64% <ø> (ø)

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