Custom Client support

[olgavrou]

Why are these changes needed?

This PR adds support for custom client calls to be made. The idea is that the user can specify their own custom client class and load it into the configuration (e.g. for loading local models). As long as they adhere to the Client class's interface and response protocol then everything should work fine with only this addition to the config (full usage shown in the unit test):

from custom_file import CustomClient

config_list = [
    {
        "model": "local_model_name",
        "custom_client" = CustomClient(),
        "device": "cuda",
        "params": {
            "max_length": 1000,
            "temperature": 1.1,
            [other params that will be consumed in the Custom Client so user has full control over what is set and what is consumed]
        }
    }
]

The PR looks big but it is mostly moving code around. If this is something that we want then I can go ahead and add docs and a notebook

Ideally the client interface would not live under a directory called oai and the client wrapper wouldn't be called OpenAIWrapper but something more generic, but that can be examined at a later date

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: 117 lines in your changes are missing coverage. Please review.

Comparison is base (a0288e0) 26.54% compared to head (756e7b8) 38.12%.

Files	Patch %	Lines
autogen/oai/client.py	27.04%	112 Missing and 4 partials ⚠️
autogen/oai/openai_utils.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #831       +/-   ##
===========================================
+ Coverage   26.54%   38.12%   +11.57%     
===========================================
  Files          28       28               
  Lines        3805     3864       +59     
  Branches      865      917       +52     
===========================================
+ Hits         1010     1473      +463     
+ Misses       2724     2263      -461     
- Partials       71      128       +57     

Flag	Coverage Δ
unittests	38.06% <27.77%> (+11.57%)	⬆️

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[sonichi]

Suggestion: use pre-commit to format the code.

[ekzhu]

I tried to merge the conflicts but don't have access to the repo. @olgavrou can merge the conflicts?

[ekzhu]

Customizable client adds a lot of value. Regarding this PR I think it moves the code to the right direction. A couple of things to consider in a broader context:

1. The current PR addresses the client API. Ideally, we want to bind message format API call parameters with the client as well. See #1129 for some issues with alternative message format.
2. How should we approach LLM serving tools that aims for OpenAI compatibility, like vllm and litellm. I think a customizable client layer in our code is still needed as we may need to move faster than dependencies.

Lastly, the PR does a lot of code moving so need to make sure the latest changes in the main branch were copied to the new destination.

[olgavrou]

I tried to merge the conflicts but don't have access to the repo. @olgavrou can merge the conflicts?

@ekzhu

I'll open a fresh PR for this since this one has gone quite out of sync with main branch, and close this one

[olgavrou]

closing in favour of #1345