Update vllm colab to work with asyncio requirements

[damccorm]

In #32687 I updated the vLLM model handler to use an asyncio event loop. This led to huge efficiency gains (around <batch_size>x faster), but also broke it in environments where there is already an asyncio event loop because of https://stackoverflow.com/questions/55409641/asyncio-run-cannot-be-called-from-a-running-event-loop-when-using-jupyter-no

This is generally fine (this is not a normal environment to run Beam), but it does break things for colab environments. It can be easily fixed with this tweak, though - this PR makes that tweak to our example so that this keeps working. I've confirmed we don't need this to run in Dataflow/other runners

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[damccorm]

This shouldn't be merged before the release, but it can get reviewed now I guess

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[damccorm]

R: @shunping

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[shunping]

LGTM!