Fix attn bug in qwen3-8b benchmark test

[PerryZhang01]

Purpose

This PR targets to fix the benchmark test of Qwen-8b. https://ontrack-internal.amd.com/browse/SWDEV-557861

Test Plan

vllm bench throughput --model Qwen/Qwen3-8B -tp 1 --num-prompts 1024 --input-len 128 --output-len 2048 --max-num-seqs 1024 --max-seq-len-to-capture 40960 --max-num-batched-tokens 40960 --max-model-len 8192 --async-scheduling --trust-remote-code --output-json pyt_vllm_qwen3-8b_throughput.json

Test Result

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Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
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