Compressible Agent require model field for llm_config

[yiranwu0]

Why are these changes needed?

Update compression notebook to add model field to llm_config to make it runnable. Update related docs.

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.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 46.20%. Comparing base (2503000) to head (a375c87).

❗ Current head a375c87 differs from pull request most recent head 28e4614. Consider uploading reports for the commit 28e4614 to get more accurate results

Files	Patch %	Lines
autogen/agentchat/contrib/compressible_agent.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1903       +/-   ##
===========================================
+ Coverage   36.13%   46.20%   +10.07%     
===========================================
  Files          63       63               
  Lines        6670     6672        +2     
  Branches     1475     1601      +126     
===========================================
+ Hits         2410     3083      +673     
+ Misses       4060     3334      -726     
- Partials      200      255       +55     

Flag	Coverage Δ
unittests	46.13% <0.00%> (+10.00%)	⬆️

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