Allow insecure gRPC connections for federated learning (#8181)

* Allow insecure gRPC connections for federated learning

* format

python-package/xgboost/federated.py

@@ -6,9 +6,9 @@ from .core import _LIB, XGBoostError, _check_call, build_info, c_str
 def run_federated_server(
     port: int,
     world_size: int,
-    server_key_path: str,
-    server_cert_path: str,
-    client_cert_path: str,
+    server_key_path: str = "",
+    server_cert_path: str = "",
+    client_cert_path: str = "",
 ) -> None:
     """Run the Federated Learning server.
 
@@ -19,22 +19,25 @@ def run_federated_server(
     world_size: int
         The number of federated workers.
     server_key_path: str
-        Path to the server private key file.
+        Path to the server private key file. SSL is turned off if empty.
     server_cert_path: str
-        Path to the server certificate file.
+        Path to the server certificate file. SSL is turned off if empty.
     client_cert_path: str
-        Path to the client certificate file.
+        Path to the client certificate file. SSL is turned off if empty.
     """
     if build_info()["USE_FEDERATED"]:
-        _check_call(
-            _LIB.XGBRunFederatedServer(
-                port,
-                world_size,
-                c_str(server_key_path),
-                c_str(server_cert_path),
-                c_str(client_cert_path),
+        if not server_key_path or not server_cert_path or not client_cert_path:
+            _check_call(_LIB.XGBRunInsecureFederatedServer(port, world_size))
+        else:
+            _check_call(
+                _LIB.XGBRunFederatedServer(
+                    port,
+                    world_size,
+                    c_str(server_key_path),
+                    c_str(server_cert_path),
+                    c_str(client_cert_path),
+                )
             )
-        )
     else:
         raise XGBoostError(
             "XGBoost needs to be built with the federated learning plugin "