[FEA]: Add function return documentation for `LLMService`

[mdemoret-nv]

Is this a new feature, an improvement, or a change to existing functionality?

Improvement

How would you describe the priority of this feature request

Low (would be nice)

Please provide a clear description of problem this feature solves

All of the docstrings in the LLMService and derived classes do not document the return types.

Describe your ideal solution

Add return types for classes derived from LLMService and LLMClient. Return type annotations should be in the numpydoc format. i.e.

        Returns
        -------
        str
            _description_

Additional context

No response

Code of Conduct

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• [X] I have searched the open feature requests and have found no duplicates for this feature request