[P/D][v1] Allow registering external kv connector from args

[KingsleyZhang123]

KVConnectorBase_V1 API is introduced in (#15960), however, due to the multi-process nature, it's hard to register a customized kv connector implementations from outside context.

This change will make the kv connector registration more flexible as it allows for external dependency injection.
Example Usage:

kv_connector_registry_args={
  "name": "CustomConnector",
  "module_path": "external.module.path",
  "class_name": "CustomConnector",
},

This will allow external developers to register customized kv connectors without keep adding a boilerplate & updating the factory.py.

The third commit moves the kv connector init after model loading, allowing vllm_config to be populated with all attention layers info.

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[liuzijing2014]

cc @njhill for review

[jiayisuse]

could this be called multiple times

[KingsleyZhang123]

No, based on current v1 design each process will only call it once

[KingsleyZhang123]

Double checked, when running world_size = 1, UniProcExecutor will be used, scheduler & worker will run on the same process and this will be called twice.

vllm/vllm/config.py

Lines 1828 to 1829 in d67085c

	if self.distributed_executor_backend is None and self.world_size == 1:
	self.distributed_executor_backend = "uni"

[maobaolong]

Interesting feature, it will help private kv connector implementation works without change or hack vllm, this feature is very useful.

[sdavidbd]

@liuzijing2014 thanks for the PR—this is definitely a step in the right direction (from the GenericKVConnector #17840) toward supporting external connector integration. I'd like to offer an alternative approach that I believe simplifies things further: #18142.

Rather than dynamically registering the external KV connector through an opaque config field (encapsulating multiple fields: connector name, module path, class name), the factory can directly load the external implementation using a single, semantically clear config field: the external module path. This eliminates the need for explicit registration or wrapper classes.

More importantly, with this approach:

• No connector registration is needed in V1
• No vendor-specific connector implementation needs to be upstreamed to vLLM
• Integration is declarative and self-contained, keeping the vLLM codebase clean
• Vendors can ship their own connector logic independently, and users can consume it by setting config only

This minimizes boilerplate and improves usability while preserving full flexibility. The supported external connectors can simply be documented or included in examples, rather than embedded in the core repo.

I’d be glad to hear your thoughts.

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @KingsleyZhang123.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

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