[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/fd_add_dummy_model_runner/__init__.py

@@ -0,0 +1,26 @@
+# 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.
+
+
+class MyModelRunner:
+    def __init__(self, rank=0) -> None:
+        super().__init__()
+        self.rank = rank
+
+    def get_rank(self):
+        return self.rank
+
+
+def get_runner():
+    return MyModelRunner