[Serve] Make batching work with multiplexing

[abrarsheikh]

fixes https://github.com/ray-project/ray/issues/56633

• [x] Add documentation
• [x] update get_multiplexed_model_id to see if we are batch context first
• [x] update logic
• [x] add tests
• [x] does not introduce any backwards incompatibility, previously the system did not provide any guarantee about contents of a batch and now we are add a constraint that guarantees each batch contains requests for same model.

The thing I dislike about this implementation is that it does not fill the batch in the case where the replica is responsible for > 2 models and incoming traffic is equally distributed between those models. Becasue the current implementation fills the batch first, then divides them.

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