[Misc] Allow initializing KV cache transfer agent when using third-party library for disaggregated prefill

[KuntaiDu]

Allow initializing the KV cache transfer agent even when kv_parallel_size==1.

This is because, when using third-party library for disaggregated prefill, it's possible that it handles everything in the library (and thus it does not need to set kv_parallel_size>1), but we still need to initialize the agent to initialize third-party connector in the first place.

[github-actions]

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