add GPT-J ONNX config to Transformers
# What does this PR do?

I'm looking for contributing to `Transformers` repository by adding more OnnxConfig to available models on the hub.
I have created a little organization [ONNXConfig for all](https://huggingface.co/OWG) to track the models that needs support for ONNX.

This is the first contribution since CamemBERT OnnxConfig some months ago.

I took example on `GPT2` and `GPT-Neo` OnnxConfig but I'm not sure if everything is good or if `GPT-J` needs special things to be added.

So this PR is a work in progress. If anyone can send me ressources to read to understand if it lacks anything, it would be awesome! :hugs: 

## Who can review?

Models GPT2 / GPT-Neo
@LysandreJik @michaelbenayoun 

_The documentation is not available anymore as the PR was closed or merged._<|||||>So what is the next step to make it merged ?<|||||>Hey @ChainYo the last thing to check is that the slow tests pass with:

```
RUN_SLOW=1 python -m pytest tests/onnx/test_onnx_v2.py -k "gpt-j"
```

We only run these on the `main` branch, so it would be good to know that they pass before we merge.

Apart from that, the PR looks really good - gently pinging @sgugger or @LysandreJik for final approval