[V1][Bugfix] Propagate V1 LLMEngine properly

[comaniac]

#9888 makes V1 LLMEngine import lazily, but failed to pass the right engine to the constructor, resulting in crashing when launching the engine.

We should really make some basic tests for v1 ASAP...

cc @joerunde @WoosukKwon @youkaichao

[github-actions]

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[njhill]

@comaniac I'm just checking whether this makes sense given that we won't use MQLLMEngine with V1.

[njhill]

Yes this won't be needed once we merge #9826 (hopefully imminently), which reverts the use of v1 engine in mutliprocessing/engine.py.

[comaniac]

Ok make sense. Than we could close this one once #9825 is merged.

[joerunde]

Ah, good catch @comaniac!

I will start enabling more tests with v1, but won't do any of the engine/multiprocessing ones since this won't be used with v1