Ability to add MessageTransforms to the GroupChat's Select Speaker nested chat (speaker_selection_method='auto')

[marklysze]

This provides the ability to add MessageTransforms to the nested select speaker chat that occurs in a GroupChat when the speaker selection method is 'auto'. The MessageTransforms apply only to the messages within that nested select speaker chat (and not to the broader GroupChat chat).

The GroupChat constructor has a new, optional, parameter called select_speaker_auto_message_transforms that takes in a MessageTransforms object, or None.

Within GroupChat, the provided MessageTransforms object will be assigned to self._auto_message_transforms and used within _auto_select_speaker and a_auto_select_speaker.

The nested select speaker chat has two agents, checking_agent and speaker_selection_agent and the MessageTransforms are added to the speaker_selection_agent as that is the agent that utilises the LLM.

I have not added documentation, yet, as I wanted to have it reviewed first.

cc @Nathan-Intergral, @WebsheetPlugin

Why are these changes needed?

These changes enable greater control and flexibility of messages for speaker selection. Particularly useful for non-OpenAI models with limited context / reasoning ability.

Related issue number

Addresses #2499.

Addresses part of #2583.

I will carry forward other suggested changes in #2583 - select_speaker_auto_compression, select_speaker_auto_token_limit, select_speaker_auto_token_limit - it is useful to have these simple methods to enable message transforms without the developer having to create them, however as I test the effectiveness of the select speaker method I need more time to think through what would be most beneficial.

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 25.00000% with 9 lines in your changes are missing coverage. Please review.

Project coverage is 16.90%. Comparing base (19de99e) to head (678f9c0).

Files	Patch %	Lines
autogen/agentchat/groupchat.py	18.18%	9 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2719       +/-   ##
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- Coverage   34.36%   16.90%   -17.46%     
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  Files          87       87               
  Lines        9336     9347       +11     
  Branches     1987     2143      +156     
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- Hits         3208     1580     -1628     
- Misses       5862     7705     +1843     
+ Partials      266       62      -204     

Flag	Coverage Δ
unittests	16.86% <25.00%> (-17.51%)	⬇️

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[ekzhu]

Looks pretty good to me

[marklysze]

I'll add a topic under Group Chat in the documentation on using transform messages for the select speaker nested chat.

[marklysze]

Okay, I've added a topic to the conversation to show how to apply Transform Messages to the speaker selection nested-chat. Let me know if there's anything else to do/change.