Add last_speaker to GroupChatManager

[thinkall]

Why are these changes needed?

Add a property last_speaker for GroupChatManger, so users can have easier access to the info of speakers. The use case is discussed in #3279

Related issue number

Closes #3279

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.

[marklysze]

Thanks @thinkall, I've tested these changes and they look good.

It may me good to add it somewhere in the documentation because I don't think it will show, even in the API reference, otherwise. The other option is to create it as a function on GroupChatManager, e.g. def get_final_speaker() -> Agent, and add a docstring. If this is done, I'd suggest changing self.last_speaker to self._last_speaker.

[thinkall]

Thanks @thinkall, I've tested these changes and they look good.

It may me good to add it somewhere in the documentation because I don't think it will show, even in the API reference, otherwise. The other option is to create it as a function on GroupChatManager, e.g. def get_final_speaker() -> Agent, and add a docstring. If this is done, I'd suggest changing self.last_speaker to self._last_speaker.

Thank you very much for the suggestions, @marklysze ! I've added the docstring. Instead of get_final_speaker, I used last_speaker, as it's a property.