PCP/DCP 适配mooncake layerwise connector

[ksiyuan]

What this PR does / why we need it?

Does this PR introduce any user-facing change?

How was this patch tested?

• vLLM version: v0.12.0
• vLLM main: vllm-project/vllm@ad32e3e

[gemini-code-assist]

Code Review

此 PR 旨在为 mooncake layerwise connector 适配 PCP/DCP。改动范围较广，并引入了处理分布式 KV 缓存传输的复杂逻辑。我发现了一些严重问题，涉及不正确的端口计算、竞争条件和错误的数据切片，这些问题可能导致通信失败或行为不正确。为了确保实现的正确性，应解决这些问题。

[gemini-code-assist]

切分 meta.local_block_ids 的逻辑不正确。在每次循环迭代中，它都重复地从列表的开头进行切片（meta.local_block_ids[:num_blocks_to_push]）。这将导致所有切分都使用相同的初始本地块集合，从而导致不正确的数据传输。您应该使用一个偏移量来在每次迭代中切分 meta.local_block_ids 的正确部分，类似于 remote_block_offset 用于 meta.remote_block_ids 的方式。

        remote_block_offset = 0
        local_block_offset = 0
        for local_kv_id in range(len(remote_handshake_port_list)):
            num_blocks_to_push = local_block_nums[local_kv_id]
            local_block_ids_list.append(
                meta.local_block_ids[local_block_offset:local_block_offset +
                                     num_blocks_to_push])
            remote_block_ids_list.append(
                meta.remote_block_ids[remote_block_offset:remote_block_offset +
                                      num_blocks_to_push])
            remote_block_offset += num_blocks_to_push
            local_block_offset += num_blocks_to_push

[github-actions]

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