fix(car): shfl and ag dispatch

[TennyWang1223]

Motivation

hip shfl runs opus::fp16_t and opus::bf16_t error
allgather dispatch logic needs a branch fallback to rccl

Technical Details

replace __shfl() by ds_bpermute
add fallback logic in framework

Test Plan

see ar and ag single test script

Test Result

rccl fallback success

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:

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• ✅ Aiter op tests (gfx942 + gfx950)
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Extended tests (opt-in via labels):

Label	Tests
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ci:atom	ATOM benchmark (DeepSeek-R1 + GPT-OSS)
ci:vllm	vLLM benchmark
ci:all	All of the above

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