fea(car): support custom group device

[TennyWang1223]

Motivation

ep_group can't rum custom allreduce
support customize communication group setup, custom can select which device in which group

Technical Details

add more if else and assert, create custom_all_reduce interface

Test Plan

see op_tests/multigpu_tests/test_custom_group_allreduce.py

Test Result

run with AITER_LOG_MORE=1 and see all launched kernels from aiter custom_all_reduce
accuracy pass, group set correct

Submission Checklist

• Look over the contributing guidelines at https://github.com/ROCm/ROCm/blob/develop/CONTRIBUTING.md#pull-requests.

[github-actions]

🏷️ CI Guide

Runs automatically on every PR:

• ✅ Pre-checks (submodule verification, code formatting)
• ✅ Aiter op tests (gfx942 + gfx950)
• ✅ Triton tests (only when aiter/ops/triton/** or related paths are changed)

Extended tests (opt-in via labels):

Label	Tests
ci:triton-355	Run Triton tests on MI355 in addition to MI325
ci:sglang	SGLang integration tests
ci:atom	ATOM benchmark (DeepSeek-R1 + GPT-OSS)
ci:vllm	vLLM benchmark
ci:all	All of the above

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