This introduces a python multiprocessing-based executor that can be used as an alternative to Ray for single-node inferencing.

With the changes in this PR, Ray will continue to be used for parallel workers if it's installed, otherwise vanilla python multiprocessing is used. It can also be overridden with --no-worker-use-ray.

The existing distributed tests have been updated to run with/without Ray.

Worker processes are shut down when the LLMEngine is garbage collected.

Co-authored-by: SAHIL SUNEJA <suneja@us.ibm.com>

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