viable/strict/1792492019: [FSDP2] Separate collective copy preparation from execution (#200034)

发生了什么

Keep the original AG/RS preparation and copying as defaults. AI-assisted implementation and summary prepared with Codex and Claude Code for the requested change above. FSDP now prepares each collective copy separately from executing it, so the next PR can register native copies. To review, start with Note [All-gather output fn] and Note [Reduce-scatter input fn] in _fsdp_collectives.py and AllGatherInput in _fsdp_ap…

摘要按规则整理自下方来源原文

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