[nixl] fix: missing metadata for handshake in multi-node (#1576)

Signed-off-by: AlpinDale <alpindale@gmail.com>

Author: AlpinDale

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

@@ -116,6 +116,15 @@ class KVConnectorRole(enum.Enum):
     WORKER = 1
 
 
+class KVConnectorHandshakeMetadata(ABC):  # noqa: B024
+    """
+    Metadata used for out of band connector handshake between
+    P/D workers. This needs to serializeable.
+    """
+
+    pass
+
+
 class KVConnectorMetadata(ABC):  # noqa: B024
     """
     Abstract Metadata used to communicate between the
@@ -314,6 +323,17 @@ def get_kv_connector_stats(self) -> Optional["KVConnectorStats"]:
         """
         return None
 
+    def get_handshake_metadata(self) -> KVConnectorHandshakeMetadata | None:
+        """
+        Get the KVConnector handshake metadata for this connector.
+        This metadata is used for out-of-band connector handshake
+        between P/D workers.
+        Returns:
+            KVConnectorHandshakeMetadata: the handshake metadata.
+            None if no handshake metadata is available.
+        """
+        return None
+
     # ==============================
     # Scheduler-side methods
     # ==============================
@@ -471,6 +491,16 @@ def build_kv_connector_stats(
         """
         return None
 
+    def set_xfer_handshake_metadata(
+        self, metadata: dict[int, KVConnectorHandshakeMetadata]
+    ) -> None:
+        """
+        Set the KV connector handshake metadata for this connector.
+        Args:
+            metadata (KVConnectorHandshakeMetadata): the handshake metadata to set.
+        """
+        return None
+
     @classmethod
     def build_prom_metrics(
         cls,