[ONNX] transformer.onnx exporting fails for longformer 
## Environment info

- `transformers` version: 4.16.0.dev0
- Platform: Linux-5.11.0-1024-gcp-x86_64-with-debian-bullseye-sid
- Python version: 3.7.11
- PyTorch version (GPU?): 1.10.1+cu102 (True)
- Tensorflow version (GPU?): 2.7.0 (True)
- Flax version (CPU?/GPU?/TPU?): not installed (NA)
- Jax version: not installed
- JaxLib version: not installed
- Using GPU in script?: Using transformer.onnx 
- Using distributed or parallel set-up in script?: No


Models:
- Longformer: @patrickvonplaten

## Information

The same error occurs for 
- allenai/longformer-base-4096
- valhalla/longformer-base-4096-finetuned-squadv1
- allenai/longformer-large-4096

## To reproduce

Steps to reproduce the behavior:

```
python -m transformers.onnx --model=allenai/longformer-base-4096  ./longformer-base-4096 

```

### Error Code 
`RuntimeError: 0INTERNAL ASSERT FAILED at "../torch/csrc/jit/ir/alias_analysis.cpp":584, please report a bug to PyTorch. We don't have an op for aten::constant_pad_nd but it isn't a special case.  Argument types: Tensor, int[], bool, `




Hi @RaedShabbir thank you for reporting this bug! It seems that this one slipped through our unit tests and it's not currently possible to export Longformer due to a limitation with the ops supported in JIT.

We plan to remove Longformer from the list of supported architectures for ONNX until a long-term solution can be found. In the meantime, if you'd like to investigate which part of the Longformer implementation is causing the error, then please feel free to comment here on what you find!<|||||>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.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md) are likely to be ignored.<|||||>Working on it <|||||>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.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md) are likely to be ignored.