fix deepspeed tests
Making deepspeed tests work again after the big tests structure reshuffle.

@LysandreJik - we can't have `tests/deepspeed/__init__.py` as then it conflicts with the actual `deepspeed` package, please see: https://github.com/huggingface/transformers/pull/15725#issuecomment-1056027398

Additionally I don't think we should turn tests into packages, as it'd cause all kinds of problems. Instead, I think we should extract any package-worthy code that is re-used by multiple tests and put it under `src/transformers` like `testing_utils.py`.

Please let me know if the failing doc builder is a false alarm. Thanks.

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_15881). All of your documentation changes will be reflected on that endpoint.