[OAI Serving] Validate greedy generation args

[jenchen13]

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What does this PR do ?

For NeMo OAI serving, validate greedy generation arguments in the request to be compatible with core-eval/nv_eval, which only exposes temperature and top_p arguments. If temperature=0 and top_p=0 in core-eval, then top_k by default is 0 in OAI serving which will cause an error in megatron-core generation.

Collection: [Note which collection this PR will affect]

Changelog

• add a Pydantic model validator for CompletionRequest which checks if temperature=0 and top_p=0 and sets top_k=1 if True.

Usage

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[marta-sd]

LGTM, thanks!

[athitten]

Thank you for the PR @jenchen13 !