[ROCm] Pass moe_buf to AITER to eliminate MoE output copy

[tpopp]

Summary

• Plumb moe_buf and moe_sorting_dispatch_policy through the vLLM AITER fused MoE interface so the kernel writes directly into the caller's output buffer, avoiding a device-to-device copy on every forward pass
• Add VLLM_ROCM_AITER_MOE_DISPATCH_POLICY environment variable to control AITER MoE sorting dispatch policy
• Depends on AITER-side change: Allow callers to pass pre-allocated moe_buf to avoid output copy ROCm/aiter#2663

Test plan

• Verify ROCm MoE models (e.g. Mixtral, Qwen3-Next) produce correct output
• Benchmark to confirm reduced latency from eliminated copy
• Verify non-ROCm paths are unaffected (new params default to no-op values)

Made with Cursor

[gemini-code-assist]

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