[KV Connector] Update lmcache kv_offloading_backend to use LMCacheMPConnector

[maobaolong]

Purpose

Switch the default kv_offloading_backend=lmcache path to LMCache's multi-process (MP) mode so that vLLM talks to a standalone LMCache server via LMCacheMPConnector instead of the legacy in-process LMCacheConnectorV1.

Use LMCacheMPConnector as the connector for --kv-offloading-backend lmcache.

Drop the now-irrelevant lmcache.local_cpu / lmcache.max_local_cpu_size extra config — KV capacity is owned by the standalone LMCache server in MP mode.

Rely on LMCacheMPConnector defaults (tcp://localhost:5555, mq_timeout=300s, heartbeat_interval=10s); users can still override via kv_connector_extra_config (lmcache.mp.host / lmcache.mp.port / ...).
Removed the unused num_kv_ranks local along the way.

Test Plan

1. Start an LMCache server listening on tcp://localhost:5555.
2. Launch vLLM with the new default path:

   vllm serve <model> \
       --kv-offloading-backend lmcache \
       --kv-offloading-size 20

Test Result

vLLM and lmcache multi-process server can start successfully, and double send a repeat request to the vllm then check the log from lmcache server to verify that chunk stored and retrieved.

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Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.

[gemini-code-assist]

Code Review

This pull request updates the lmcache backend configuration in vllm/config/vllm.py by replacing LMCacheConnectorV1 with LMCacheMPConnector. This change transitions the system to use multi-process mode, where KV storage capacity is managed by a standalone LMCache server process. Consequently, the calculation for num_kv_ranks and the explicit configuration of local CPU size have been removed. I have no feedback to provide as there were no review comments.

[maobaolong]

@ApostaC This is the PR updated the kv_offloading_backend to use LMCacheMPConnector by default. PTAL

[ApostaC]

This one looks good to me! @maobaolong, please make sure that the LMCache side has enough information to avoid the user's confusion.

[maobaolong]

This one looks good to me! @maobaolong, please make sure that the LMCache side has enough information to avoid the user's confusion.

@ApostaC Thanks for your review, will create a PR on lmcache side to give a warn if the lmcache mp server cannot reach.

[maobaolong]

LMCache/LMCache#3365 @ApostaC The lmcache-side PR is here, please help to take a look when you have time, thanks!

[ApostaC]

@maobaolong UT seems to be failing: