[BugFix][XPU] Fix kv_cache management bug

[ddchenhao66]

Motivation

当xpu开启attention store时，目前会存在在model runner中重复create kv cache的过程，需要解决重复分配cache问题

Modifications

在xpu model runner里修改create kv cache的条件逻辑

Usage or Command

Accuracy Tests

Checklist

• Add at least a tag in the PR title.
  • Tag list: [[FDConfig],[APIServer],[Engine], [Scheduler], [PD Disaggregation], [Executor], [Graph Optimization], [Speculative Decoding], [RL], [Models], [Quantization], [Loader], [OP], [KVCache], [DataProcessor], [BugFix], [Docs], [CI], [Optimization], [Feature], [Benchmark], [Others], [XPU], [HPU], [GCU], [DCU], [Iluvatar], [Metax]]
  • You can add new tags based on the PR content, but the semantics must be clear.
• Format your code, run pre-commit before commit.
• Add unit tests. Please write the reason in this PR if no unit tests.
• Provide accuracy results.
• If the current PR is submitting to the release branch, make sure the PR has been submitted to the develop branch, then cherry-pick it to the release branch with the [Cherry-Pick] PR tag.

[paddle-bot]

Thanks for your contribution!

[PaddlePaddle-bot]

📋 Review 摘要

PR 概述：修复 XPU model runner 中 KV cache 重复创建的问题

变更范围：fastdeploy/worker/xpu_model_runner.py

影响面 Tag：[XPU] [KVCache]

📝 PR 规范检查

✅ PR 标题和描述符合规范，包含必要的 [BugFix] 和 [XPU] 标签。

问题

未发现阻塞性问题。

总体评价

本次修改将 XPU model runner 的 KV cache 创建逻辑与 GPU/Metax 标准实现对齐。原逻辑只检查 splitwise_role == "mixed"，忽略了 num_cpu_blocks 和 kvcache_storage_backend 两个条件，导致在开启 attention store 时会重复创建 KV cache。修改后的逻辑正确地修复了这个问题，代码简洁且符合项目规范。

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files

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[hong19860320]

LGTM