Add documentation and tests for the LLMService

[mdemoret-nv]

As part of Sherlock, a prototype of the LLMService was created. This class will need the following improvements before being merged back to branch-23.11:

Tasks

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1. Add docstrings for the class (including when it should be used and what task it performs)
2. Add docstrings to each public method
3. Add unit tests for all public functions which execute all code paths
4. Add an integration test to verify the component works as part of a pipeline

You can find the prototype located here

Note: the location above links directly to a commit with the prototype. The current location and functionality of the class may have changed.

[dagardner-nv]

Duplicate of #1303