Remove chunked prefill length divide for DP

[jokerwyt]

Motivation

Modifications

After removing that division, my test on DeepSeek-R1 ran smoothly for about avg 15K, max 30K input benchmark, DP=4 and TP=16, chunksz 32768.
Call for more test for it.

Related #4390

Checklist

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[merrymercy]

I think we still need this division. This is because we want to make the activation memory the same for TP=8 and DP=8. For your use case, can you manually adjust --chunked-prefill-size when you launch the server?