Fix wrong model version, it's not 12 (the onnx_opset_version()), not 11 (the opset version of the latest stable), but 10 (#2478)

Co-authored-by: Ke Zhang <kezhan@microsoft.com>

Author: daquexian

onnx/backend/test/case/model/__init__.py

@@ -35,8 +35,9 @@ def expect(model,  # type: ModelProto
         ))
 
 
+base_model_opset_version = 10
 BASE_URL = 'https://s3.amazonaws.com/download.onnx/models/opset_{}'.format(
-    onnx.defs.onnx_opset_version())
+    base_model_opset_version)
 
 
 def collect_testcases():  # type: () -> List[TestCase]