Add onnx config for RoFormer
Author: skrsna
Pull request: yes

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Comments:
Hey nice PR! Did you try to convert one `RoFormer` model with your add ?
If so you could add the converted model to the [ONNXConfig for all](https://huggingface.co/OWG) organization, it would be awesome!

_The documentation is not available anymore as the PR was closed or merged._

> Hey nice PR! Did you try to convert one `RoFormer` model with your add ? If so you could add the converted model to the [ONNXConfig for all](https://huggingface.co/OWG) organization, it would be awesome!

```
python -m transformers.onnx -m junnyu/roformer_chinese_sim_char_ft_small onnx/model.onnx
Validating ONNX model...
	-[✓] ONNX model output names match reference model ({'last_hidden_state'})
	- Validating ONNX Model output "last_hidden_state":
		-[✓] (2, 8, 384) matches (2, 8, 384)
		-[✓] all values close (atol: 1e-05)
All good, model saved at: onnx/model.onnx
``` 
:) 

It seems that there is 2x `sequence-classification` for features

> All good, model saved at: onnx/model.onnx

Nice news!

> Thanks for adding RoFormer @skrsna and thank you to @ChainYo for the review!
> 
> I agree with @ChainYo's suggestions and would also like to see this included in the slow tests if possible. Could you add a roformer checkpoint to `text_onnx_v2.py` please, e.g. maybe this works well: https://huggingface.co/junnyu/roformer_chinese_base
> 
> Then please run
> 
> ```
> RUN_SLOW=1 pytest tests/onnx/test_onnx_v2.py -s -k "roformer"
> ```

Thanks for the review @ChainYo and @lewtun, I added roformer to the onnx test case 

```
tests/onnx/test_onnx_v2.py::OnnxExportTestCaseV2::test_pytorch_export_65_roformer_causal_lm If you want to use `RoFormerForCausalLM` as a standalone, add `is_decoder=True.`
PASSED
tests/onnx/test_onnx_v2.py::OnnxExportTestCaseV2::test_pytorch_export_66_roformer_default PASSED
tests/onnx/test_onnx_v2.py::OnnxExportTestCaseV2::test_pytorch_export_67_roformer_masked_lm PASSED
tests/onnx/test_onnx_v2.py::OnnxExportTestCaseV2::test_pytorch_export_68_roformer_multiple_choice PASSED
tests/onnx/test_onnx_v2.py::OnnxExportTestCaseV2::test_pytorch_export_69_roformer_question_answering PASSED
tests/onnx/test_onnx_v2.py::OnnxExportTestCaseV2::test_pytorch_export_70_roformer_sequence_classification PASSED
tests/onnx/test_onnx_v2.py::OnnxExportTestCaseV2::test_pytorch_export_71_roformer_token_classification PASSED

```