Support onnx in tritonserve

Signed-off-by: Jael Gu <mengjia.gu@zilliz.com>

auto_transformers.py

@@ -229,6 +229,10 @@ class AutoTransformers(NNOperator):
         else:
             return ['pytorch']
 
+    @property
+    def supported_formats(self):
+        return ['onnx']
+
     @staticmethod
     def supported_model_names(format: str = None):
         full_list = [