Demo of federated learning using NVFlare (#7879)

Co-authored-by: jiamingy <jm.yuan@outlook.com>

demo/nvflare/config/config_fed_client.json

@@ -0,0 +1,22 @@
+{
+  "format_version": 2,
+  "executors": [
+    {
+      "tasks": [
+        "train"
+      ],
+      "executor": {
+        "path": "trainer.XGBoostTrainer",
+        "args": {
+          "server_address": "localhost:9091",
+          "world_size": 2,
+          "server_cert_path": "server-cert.pem",
+          "client_key_path": "client-key.pem",
+          "client_cert_path": "client-cert.pem"
+        }
+      }
+    }
+  ],
+  "task_result_filters": [],
+  "task_data_filters": []
+}

demo/nvflare/config/config_fed_server.json

@@ -0,0 +1,22 @@
+{
+  "format_version": 2,
+  "server": {
+    "heart_beat_timeout": 600
+  },
+  "task_data_filters": [],
+  "task_result_filters": [],
+  "workflows": [
+    {
+      "id": "server_workflow",
+      "path": "controller.XGBoostController",
+      "args": {
+        "port": 9091,
+        "world_size": 2,
+        "server_key_path": "server-key.pem",
+        "server_cert_path": "server-cert.pem",
+        "client_cert_path": "client-cert.pem"
+      }
+    }
+  ],
+  "components": []
+}