feat: add StatelessProcessGroup to extend collective library

[kip-cxj]

Motivation

Add stateless communication group. To enable more flexible creation of communication groups and resolve compatibility issues with other programs that also use the torch.distributed.
Current support vllm, while sglang does not yet supprt pyhccl. Which feature depends on add pyhccl in sglang.
If the current approach in accptable, we will provide sglang version soon.

[x1314aq]

@weixiao-huang @HubertZhang pls review this PR

test both on npu and cuda.

Model	Device Info	device_type	GatherMetas	Update (Broadcast)	Update (P2P)
Qwen3-8b	8xNvidia-A100 TP4	cuda	0.01s	1.28s (1.46GiB)	7.81s (1.72GiB)
Qwen3-8b	8xAscend-A3 TP4	npu	0.02s	1.37s (1.59GiB)	2.02s (1.47GiB)

test the same model using default torch.distributed module.

Model	Device Info	device_type	GatherMetas	Update (Broadcast)	Update (P2P)
Qwen3-8b	8xNvidia-A100 TP4	torch	0.01s	1.15s (1.46GiB)	7.68s (1.71GiB)
Qwen3-8b	8xAscend-A3 TP4	torch	0.03s	1.44s (1.59GiB)	3.83s (1.46GiB)

[weixiao-huang]

It seems this PR should depend on vLLM, this is so heavy and not an elegant way. I think ps.py should be a lightweight component, which may not depend on other heavy framework

[hanhan-networking]

It seems this PR should depend on vLLM, this is so heavy and not an elegant way. I think ps.py should be a lightweight component, which may not depend on other heavy framework

默认的还是通信方式还是torch.distributed诶，只有需要跨资源的时候才需要用到StatelessProcessGroup，如果不支持这个的话，没法合入到verl呀😆 ，不支持训推分离的架构

[HubertZhang]

是否应当设计一个 protocol DistrubutedLib，给 ps 传入一个 dist: DistributedLib 比较好一些？目前这个 import 的写法感觉隔离的还不太够？