[Core] Prefill Only Tokens Without KV Cache in Batch Requests (Disagg Prefill)

[Shaoting-Feng]

This ticket is part of [RFC]: Disaggregated Prefilling and KV Cache Transfer Roadmap #10818, with the following issue: "Adaptivity and Fault Tolerance: [Perf] If not all KV caches in the batch are received, only perform prefilling on those tokens without KV cache."

When there are multiple requests in a batch, the decode node may only receive the KV cache for some of them from the prefill node. Previously, in such cases, the decode node would perform prefilling for all requests in the batch, even if it had already received the KV cache for some requests.

This PR rebuilds the model input when the KV cache for some requests in the batch is missing, thereby preventing prefilling on those requests.

[github-actions]

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