[P/D][V1] Support dynamic loading of external KV connector implementations (#18142)

Signed-off-by: David Ben-David <davidb@pliops.com>
Co-authored-by: David Ben-David <davidb@pliops.com>

vllm/config.py

@@ -3524,6 +3524,10 @@ class KVTransferConfig:
     kv_connector_extra_config: dict[str, Any] = field(default_factory=dict)
     """any extra config that the connector may need."""
 
+    kv_connector_module_path: Optional[str] = None
+    """The Python module path to dynamically load the KV connector from.
+    Only supported in V1."""
+
     def compute_hash(self) -> str:
         """
         WARNING: Whenever a new field is added to this config,

vllm/distributed/kv_transfer/kv_connector/factory.py

@@ -58,8 +58,17 @@ class KVConnectorFactory:
             raise ValueError("Attempting to initialize a V1 Connector, "
                              f"but found {envs.VLLM_USE_V1=}")
 
-        connector_name = config.kv_transfer_config.kv_connector
-        connector_cls = cls._registry[connector_name]()
+        kv_transfer_config = config.kv_transfer_config
+        connector_name = kv_transfer_config.kv_connector
+        if connector_name in cls._registry:
+            connector_cls = cls._registry[connector_name]()
+        else:
+            connector_module_path = kv_transfer_config.kv_connector_module_path
+            if connector_module_path is None:
+                raise ValueError(
+                    f"Unsupported connector type: {connector_name}")
+            connector_module = importlib.import_module(connector_module_path)
+            connector_cls = getattr(connector_module, connector_name)
         assert issubclass(connector_cls, KVConnectorBase_V1)
         logger.info("Creating v1 connector with name: %s", connector_name)
         # NOTE(Kuntai): v1 connector is explicitly separated into two roles.