nvtx markers for async llm engine

[terrykong]

currently only the sync vllm worker has nvtx markers. this issue tracks adding this to the async llm engine for parity

[guyueh1]

@youngeunkwon0405 is it ok to re-assign this to you, as what we discussed: add coarse-grained markers in a sync function to wrap the entire generate phase, you will implement it?

[youngeunkwon0405]

I tried but haven't fully succeeded yet. You can assign this to me, but I don't think this is a prioritized item that could be delayed.