Skip onnx test cases if no onnx (apache#9272)

This was missing a guard which meant VS Code errored on test collection.

tests/python/nightly/quantization/test_quantization_accuracy_for_vit.py

@@ -14,15 +14,20 @@
 # KIND, either express or implied.  See the License for the
 # specific language governing permissions and limitations
 # under the License.
-import tvm
 import os
 import sys
-from tvm import relay
-from tvm.relay import quantize as qtz
 import logging
+
+import pytest
+
+pytest.importorskip("onnx")
+
 import onnx
+
+import tvm
+from tvm import relay
+from tvm.relay import quantize as qtz
 import tvm.testing
-import mxnet as mx
 from test_quantization_accuracy import Config, get_val_data, eval_acc
 
 logging.basicConfig(level=logging.INFO)