[FIX] force spawning pool for MacOS
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/datasets/pr_7778). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update. After more discussions on slack, we can switch the default to spawn.

Let's use multiprocess instead of multiprocessing and maybe add a check to apply this only on Macos ?
This PR gets multiprocessing to work on mac os:

```python
from datasets import load_dataset

ds = load_dataset("fka/awesome-chatgpt-prompts", split="train").take(100)

ds = ds.map(lambda x: x, num_proc=4)

ds.push_to_hub("burtenshaw/dataset-test", num_proc=4)
```