[Feature] Pluggable platform-specific scheduler

[yannicks1]

This PR enables different platforms to plugin their (hardware) specific scheduler class in vLLM version V0 and therefore addresses the scheduler part of this RFC #11162. A pluggable scheduler is needed to add support for the IBM Spyre accelerator #9652.
Note that this feature is under development for V1 but missing in the current V0.

Changes

• adding attribute scheduler_cls to class ParallelConfig in vllm/config.py
• loading Scheduler based on scheduler_cls in vllm/engine/llm_engine.py

Remarks

• since the scheduler_cls is initialized with "vllm.core.scheduler.Scheduler", the default behaviour does not change. However, the change allows to point to a (hardware) specific scheduler class in platforms.
• I opted for initialization with "vllm.core.scheduler.Scheduler" and not "auto" as it is done for attribute worker_cls to have minimal changes (Otherwise I would have to overwrite "auto" for all platforms using the default scheduler).

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[comaniac]

While we need more discussions about this feature in v1, I think it's ok for v0 to have it. Could you add a unit test with a dummy scheduler to 1) test the functionality, and 2) be an example?

[yannicks1]

Thanks for your feedback @comaniac ! I have added a test which validates the functionality.

[comaniac]

It would be good to avoid a duplicated dummy platform in the plugin tests. Can we do either of the following:

1. Do not use a dummy platform in the dummy scheduler plugin. This also demonstrates that the pluggable scheduler is compatible with existing platforms.
2. If (1) is not feasible, we could consider extending dummy platform plugin to also have a dummy scheduler, so that it becomes vllm_add_dummy_platform_and_scheduler.