use `pytest_num_workers=8` for `torch_job` and `tf_job`
# What does this PR do?

We set `pytest_num_workers` to `3` for `torch_job` and `6` for `tf_job` to avoid OOM. With the recent efforts of reducing model size in CI, we can actually set `pytest_num_workers=8`.

The full suite: all 3 jobs (PT/TF/Flax) 12-15 minutes
(on the latest nightly CI without all PRs merged today: PT: 37 min | TF: 25 min | Flax: 20 min)

The `torch_job` reach 95% of RAM (peak), and `tf_job` is at 80% of RAM. The `torch_job` with `n8` is a bit dangerous, but I think I have a way to further improvement in follow PR(s).



The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_25273). All of your documentation changes will be reflected on that endpoint.