[bugfix] adapt to new implemented get_kv_cache_spec in cpuoffload connector

[lidenghui1110]

What this PR does / why we need it?

func get_kv_cache_spec in model_runner changed a lot and caused error in cpuoffloading connector which is copied from model_runner, this PR adapts to new implemented get_kv_cache_spec to fix it.

Does this PR introduce any user-facing change?

How was this patch tested?

• vLLM version: v0.11.0
• vLLM main: vllm-project/vllm@2918c1b

[github-actions]

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[lidenghui1110]

@wangxiyuan please take a look at this bugfix, thanks.