[serve.llm] Dimensions api of embedding req does not work for serve.llm

[DriverSong]

Description

Since vllm 16331, embedding API dimensions has been supported by vllm. Ray.serve.llm also provides the dimensions api while it does not work.

Fixed in #54499

Use case

[eicherseiji]

Hi @DriverSong, thanks for filing and contributing the fix! I saw the discussion with @kouroshHakha on #54499. It seems like this class of issues should be resolved by #54554.

Can you give our nightly build a try and let us know if the fix is working as expected?