bugfix: fix DeepSeek failing to run when enabling DP.

[DongheJin]

No description provided.

[gemini-code-assist]

Code Review

This pull request fixes a bug where DeepSeek models fail when data parallelism (DP) is enabled. The fix involves remapping CHUNKED_PREFILL to PREFILL in the LLM engine when preparing inputs for DP ranks, which avoids an unstable code path. While the fix is effective, it's a workaround that lacks explanation. I've added a high-severity comment recommending the addition of a code comment to document this workaround for future maintainability. A minor log message improvement in the DeepSeek decoder is also included.