fix some cython code

engines/inference_engine.pyx

@@ -2,6 +2,18 @@ cdef class InferenceEngine:
     def __init__(self, model_bytes: bytes, batch_size: int = 1, **kwargs):
         self.batch_size = batch_size
 
+    @property
+    def engine_name(self):
+        return "onnx"
+
+    @staticmethod
+    def get_engine_filename():
+        return None
+
+    @staticmethod
+    def convert_from_onnx(bytes onnx_bytes):
+        return onnx_bytes
+
     cdef tuple get_input_shape(self):
         raise NotImplementedError("Subclass must implement get_input_shape")