ONNX export outputs many warnings

I was testing ONNX export via your **04-onnx-export.ipynb** notebook and when calling `!python -m transformers.convert_graph_to_onnx --framework pt --model bert-base-cased --opset 11 --quantize onnx/bert-base-cased2.onnx` I get many Warnings like:
```
Warning: Unsupported operator LayerNormalization. No schema registered for this operator.
Warning: Unsupported operator Attention. No schema registered for this operator.
Warning: Unsupported operator LayerNormalization. No schema registered for this operator.
Warning: Unsupported operator Gelu. No schema registered for this operator.
...
```

They only appear when using `--quantize` flag.

I know it is just a warning ... but still ... does it affect the exporting process in any way? 
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