[PD] Support PD disaggregation with Prefill PP (#8846)

Signed-off-by: Shangming Cai <caishangming@linux.alibaba.com>
Signed-off-by: Shangming Cai <csmthu@gmail.com>
Co-authored-by: root <huzhiyuan@xiaohongshu.com>
Co-authored-by: Ying Sheng <sqy1415@gmail.com>
Co-authored-by: Francis <38564764+ssssnow@users.noreply.github.com>
Co-authored-by: zitto <zhjc1124@gmail.com>

python/sglang/srt/disaggregation/base/conn.py

@@ -30,6 +30,7 @@ class KVArgs:
     # for pp prefill
     prefill_pp_size: int
     pp_rank: int
+    prefill_start_layer: int
     # for system dp
     system_dp_rank: int