### What this PR does / why we need it?
Currently, users have to set `HCCL_BUFFSIZE` to 512~1024 to perform mc2
operators (dispatch and combine) while running moe models with large
`ep_size` and `batch_size`. This environmental variable not only affects
allocated VRAM for mc2 group, but also increases VRAM allocation for dp,
tp & ep groups, leading to significant kvcache and free_memory drops.
This PR supports to automatically calculate and set `hccl_buffer_size`
for each process group **(except mc2 group)** separately when users set
`HCCL_BUFFSIZE` for mc2 group. This can significantly reduce wasted
buffer_size set for dp, tp & ep groups.
Note that current mc2 operators can only perform communication space
partitioning based on `HCCL_BUFFSIZE` configuration. Once they support
`hccl_buffer_size` configuration with `pg_options` while initializing
process group, we'll caculate the required buffer size and users would
avoid set `HCCL_BUFFSIZE` themselves.
### Does this PR introduce _any_ user-facing change?
No.
### How was this patch tested?
We performed E2E serving with deepseek_r1 initializing DP/TP/EP/MC2
process group and observed significant kv_cache and free_memory
increase!
- vLLM version: v0.11.0rc3
- vLLM main: https://github.com/vllm-project/vllm/commit/v0.11.0
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Signed-off-by: linfeng-yuan <1102311262@qq.com>
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