[Qwen3.5_dev] Cherry-pick fix for custom all-reduce - #4547
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Without this barrier, a fast rank can exit the kernel while a slow rank is still reading peer input via IPC. The fast rank's caller can then overwrite the input slot (e.g. PyTorch graph_pool reuse in a captured CUDA graph), causing the slow rank to read garbage and produce NaN / Inf in the AR output. All other AR kernels in this file already call end_sync before exit.
Co-authored-by: ColorsWind <14761584+ColorsWind@users.noreply.github.com>
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Motivation
Cherry pick 2 PRs from main branch to prevent race condition.
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