[plugin] Custom model_runner/model support (#3186)

* support custom model&&model_runner

* fix merge

* add test && update doc

* fix codestyle

* fix unittest

* load model in rl

test/plugins/test_model_runner_register.py

@@ -0,0 +1,35 @@
+# Copyright (c) 2025 PaddlePaddle Authors. All Rights Reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import unittest
+
+from fastdeploy.plugins import load_model_runner_plugins
+
+
+class TestModelRunnerRegistryPlugins(unittest.TestCase):
+    def test_model_runner_callable(self):
+        runner_class = load_model_runner_plugins()
+        device_id = 1
+
+        # create runner
+        runner = runner_class(device_id)
+
+        # test func
+        res = runner.get_rank()
+
+        self.assertEqual(res, device_id)
+
+
+if __name__ == "__main__":
+    unittest.main()