Remove usage of deprecated _serialize (apache#33000)

* Remove usage of deprecated _serialize

* Correct assignment

* indentation

* lint

* fmt

* fmt

sdks/python/apache_beam/ml/inference/onnx_inference.py

@@ -116,7 +116,15 @@ def load_model(self) -> ort.InferenceSession:
     # when path is remote, we should first load into memory then deserialize
     f = FileSystems.open(self._model_uri, "rb")
     model_proto = onnx.load(f)
-    model_proto_bytes = onnx._serialize(model_proto)
+    model_proto_bytes = model_proto
+    if not isinstance(model_proto, bytes):
+      if (hasattr(model_proto, "SerializeToString") and
+          callable(model_proto.SerializeToString)):
+        model_proto_bytes = model_proto.SerializeToString()
+      else:
+        raise TypeError(
+            "No SerializeToString method is detected on loaded model. " +
+            f"Type of model: {type(model_proto)}")
     ort_session = ort.InferenceSession(
         model_proto_bytes,
         sess_options=self._session_options,