Skip RoFormer ONNX test if rjieba not installed
# What does this PR do?

This PR adds the `@require_rjieba` decorator to the slow ONNX tests to deal with the following error in our daily CI runs:

```
(line 164)  ImportError: You need to install rjieba to use RoFormerTokenizer. See https://pypi.org/project/rjieba/ for installation.
```

~~I wasn't sure if `rjieba` should actually be installed in the GitHub workflow, but it doesn't seem to be the case for the RoFormer tests and so I omitted that for now.~~

Edit: I've added `rjieba` to the `"tests"` extras and also tested that the slow ONNX test passes when this dep is installed:

```
RUN_SLOW=1 pytest tests/onnx/test_onnx_v2.py -s -k "roformer"
```
_The documentation is not available anymore as the PR was closed or merged._<|||||>So this test is then currently just skipped on our ONNX tests? 

Should we maybe not better add `rjieba` to the test package to the test RoFormer (I couldn't find it in the `setup.py` or in the Docker file) cc @LysandreJik @sgugger <|||||>It looks like RoFormer tokenization is completely untested yes, so this package should be added in the `"testing"` extra.<|||||>I agree we should include `rjieba` for the tests, unless there are reasons for not adding specific packages.

### Further remark
If we could not add `rjieba`, it is not a good idea to add `@require_rjieba` for `test_pytorch_export`, otherwise this test won't be run for other models neither. In this case, I think we might need a specific way to skip this test for `RoFormer` (and others that require `rjieba`).<|||||>Thanks for the feedback - I'll add `rjieba` to our testing suite as well :)<|||||>Hey @sgugger @patrickvonplaten I'm hitting some peculiar issues with 2 of the slow tests of the RoFormer tokenizer. Would you mind taking a look and seeing if my decision to skip them is valid?<|||||>I'll let @patrickvonplaten decide as I know nothing on that model too :-)<|||||>There is a test dedicated to custom tokenizers with specific dependencies: https://github.com/huggingface/transformers/blob/main/.circleci/config.yml#L538

It installs `jieba` but not `rjieba`. Would it make sense to add it there? If you're testing for ONNX, it's very likely that it does not make sense as it's limited to tokenizer tests right now.<|||||>> There is a test dedicated to custom tokenizers with specific dependencies: https://github.com/huggingface/transformers/blob/main/.circleci/config.yml#L538
> 
> It installs `jieba` but not `rjieba`. Would it make sense to add it there? If you're testing for ONNX, it's very likely that it does not make sense as it's limited to tokenizer tests right now.

Thanks for the tip! Done in [3cafcb2](https://github.com/huggingface/transformers/pull/16981/commits/3cafcb2e06bc7caf0eba2e03e817fedcf0cfe073)<|||||>Hey @patrickvonplaten @LysandreJik I think this PR is ready for a final pass :)

The failing test is unrelated to the PR itself (a failing Pegasus generate test)