XGLM onnx support
### Feature request

I am trying to convert an XGLM based model using the following command, but am receiving an error that onnx does not support an XGLM based model:
```
./venv/bin/python -m transformers.onnx --model=facebook/incoder-1B onnx/
```

Error:
```
KeyError: "xglm is not supported yet. Only ['albert', 'bart', 'beit', 'bert', 'big-bird', 'bigbird-pegasus', 'blenderbot', 'blenderbot-small', 'camembert', 'convbert', 'data2vec-text', 'deit', 'distilbert', 'electra', 'flaubert', 'gpt2', 'gptj', 'gpt-neo', 'ibert', 'layoutlm', 'marian', 'mbart', 'mobilebert', 'm2m-100', 'roberta', 'roformer', 't5', 'vit', 'xlm-roberta'] are supported. If you want to support xglm please propose a PR or open up an issue."
```
cc @lewtun<|||||>Hey @FrankHeijden, indeed this architecture is not yet supported in the ONNX exporter. If you'd like to have a go at it yourself, you can follow [this guide](https://huggingface.co/docs/transformers/v4.19.2/en/serialization#exporting-a-model-for-an-unsupported-architecture) and use the `BartOnxxConfig` as a template to work from (I think this model should be similar)<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

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