### What this PR does / why we need it? Add the description for 2P1D, keeping it consistent with the content in the dev branch. ### Does this PR introduce _any_ user-facing change? no - vLLM version: v0.12.0 - vLLM main: https://github.com/vllm-project/vllm/commit/v0.12.0 Signed-off-by: mazhixin000 <mazhixinkorea@163.com> Co-authored-by: wangxiyuan <wangxiyuan1007@gmail.com>