fix(vllm_omni): default collective group communication backend

[lrq619]

修复 actor_train（FSDP2）与 actor_infer（vllm_omni）的 device_mapping 不完全相同时，权重同步通信组初始化卡住的问题。该场景需要通过 broadcast 同步权重，但原先 VllmOmniStrategy.setup_collective_group 未补齐 backend 参数，导致推理 worker 无法加入通信组，训练端一直阻塞在 rendezvous。
本次将该方法改为显式签名，增加 backend=None 默认值；未指定 backend 时使用 current_platform.communication_backend，并以关键字参数向模型转发全部六个参数。保留原有 hasattr 判断，worker extension 和调用方无需修改。

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.

---

lrq619 seems not to be a GitHub user. You need a GitHub account to be able to sign the CLA. If you have already a GitHub account, please add the email address used for this commit to your account.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}