[RL] update weight when pp_size > 1

[xiuhu17]

This PR fixes SGLang online weight update when pp_size > 1.

Problem

Weight update previously indexed payloads and distributed ranks only by tp_rank.
With pipeline parallelism, different PP stages could use the wrong tensor payload or join update collectives with incorrect ranks.

Fix

Use PP-aware rank mapping:

rank = tp_size * pp_rank + tp_rank

Changes

• Serialize tensor payloads for tp_size * pp_size ranks.
• Read update payloads with tp_size * pp_rank + tp_rank.
• Initialize weight update process groups with PP-aware rank.
• Forward weight update requests across PP stages before processing them locally.

Covered Requests

  UpdateWeightsFromDistributedReqInput
  UpdateWeightsFromTensorReqInput
  UpdateWeightsFromIPCReqInput

---

CI States

Latest PR Test (Base): ❌ Run #26910932808
Latest PR Test (Extra): ❌ Run #26910931808

[gemini-code-assist]

Warning

You have reached your daily quota limit. Please wait up to 24 hours and I will start processing your requests again!

[ShangmingCai]

Will test this change tomorrow. Others look good.

[xiuhu17]

Thanks for reviewing!