[BugFix][KVConn] Fix use of `get_required_kvcache_layout` (#22734)

Signed-off-by: Nick Hill <nhill@redhat.com>

vllm/distributed/kv_transfer/kv_connector/v1/base.py

@@ -325,4 +325,8 @@ class KVConnectorBase_V1(ABC):
             str: the required KV cache layout. e.g. HND, or NHD.
             None if the connector does not require a specific layout.
         """
+
+        if cls is KVConnectorBase_V1:
+            raise TypeError("get_required_kvcache_layout should not be called "
+                            "on the abstract base class")
         return None

vllm/distributed/kv_transfer/kv_connector/v1/multi_connector.py

@@ -228,9 +228,10 @@ class MultiConnector(KVConnectorBase_V1):
         for ktc in ktcs:
             kv_transfer_config = KVTransferConfig(**ktc)
             temp_vllm_config.kv_transfer_config = kv_transfer_config
+            connector_cls = KVConnectorFactory.get_connector_class(
+                kv_transfer_config)
             required_kvcache_layout = (
-                KVConnectorBase_V1.get_required_kvcache_layout(
-                    temp_vllm_config))
+                connector_cls.get_required_kvcache_layout(temp_vllm_config))
             if required_kvcache_layout is not None:
                 layouts.add(required_kvcache_layout)