fix: add --exclusive to MI300X SLURM salloc for accurate benchmarks

[cquil11]

Summary

• Add --exclusive flag to salloc in runners/launch_mi300x-amds.sh to prevent node sharing during benchmarks
• Only non-TP8 configs are affected (TP8 already uses all GPUs); perf-changelog updated for gptoss-fp4-mi300x-vllm and minimaxm2.5-fp8-mi300x-vllm
• NVIDIA runners already use --exclusive; this brings MI300X to parity

Test plan

• Run a non-TP8 MI300X benchmark (e.g. gptoss-fp4-mi300x-vllm TP4) and verify results are consistent with exclusive node access

🤖 Generated with Claude Code

[github-actions]

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[cquil11]

note needed
oversubscriptin is disabled by default