[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

fastdeploy/rl/rollout_model.py

@@ -56,6 +56,9 @@ class RolloutModel(nn.Layer):
     def _init_model(self) -> nn.Layer:
         """Load model from loader based on config."""
         context = paddle.LazyGuard()
+        from fastdeploy.plugins.model_register import load_model_register_plugins
+
+        load_model_register_plugins()
         architectures = f"{self.fd_config.model_config.architectures[0]}RL"
         with context:
             model_cls = ModelRegistry.get_class(architectures)