fix: Allow users to set `--kv-transfer-config` for vLLM (#4317)

Signed-off-by: Joe Chandler <jchandler@glia-ai.com>

components/src/dynamo/vllm/args.py

@@ -282,19 +282,6 @@ def parse_args() -> Config:
                 f"Please ensure the file exists and the path is correct."
             )
 
-    # Check for conflicting flags
-    has_kv_transfer_config = (
-        hasattr(engine_args, "kv_transfer_config")
-        and engine_args.kv_transfer_config is not None
-    )
-    has_connector_flag = args.connector is not None
-
-    if has_kv_transfer_config and has_connector_flag:
-        raise ValueError(
-            "Cannot specify both --kv-transfer-config and --connector flags"
-        )
-
-    if has_connector_flag:
     normalized = [c.lower() for c in args.connector]
 
     invalid = [c for c in normalized if c not in VALID_CONNECTORS]
@@ -303,13 +290,25 @@ def parse_args() -> Config:
             f"Invalid connector(s): {', '.join(invalid)}. Valid options are: {', '.join(sorted(VALID_CONNECTORS))}"
         )
 
-        if "none" in normalized or "null" in normalized:
+    # Check for custom kv_transfer_config
+    has_kv_transfer_config = (
+        hasattr(engine_args, "kv_transfer_config")
+        and engine_args.kv_transfer_config is not None
+    )
+
+    if not normalized or "none" in normalized or "null" in normalized:
         if len(normalized) > 1:
             raise ValueError(
                 "'none' and 'null' cannot be combined with other connectors"
             )
         config.connector_list = []
     else:
+        # Check for conflicting flags
+        if has_kv_transfer_config:
+            raise ValueError(
+                "Cannot specify both --kv-transfer-config and --connector flags"
+            )
+
         config.connector_list = normalized
 
     if config.engine_args.block_size is None:
@@ -433,7 +432,12 @@ def create_kv_transfer_config(config: Config) -> Optional[KVTransferConfig]:
 def overwrite_args(config):
     """Set vLLM defaults for Dynamo."""
 
-    if config.has_connector("nixl"):
+    if config.has_connector("nixl") or (
+        # Check if the user provided their own kv_transfer_config
+        config.engine_args.kv_transfer_config is not None
+        # and the connector is NixlConnector
+        and config.engine_args.kv_transfer_config.kv_connector == "NixlConnector"
+    ):
         ensure_side_channel_host()
 
     defaults = {