save_to_disk() freezes when saving on s3 bucket with multiprocessing
### Describe the bug

I'm trying to save a `Dataset` using the `save_to_disk()` function with:
- `num_proc > 1`
- `dataset_path` being a s3 bucket path e.g. "s3://{bucket_name}/{dataset_folder}/"

The hf progress bar shows up but the saving does not seem to start. 
When using one processor only (`num_proc=1`), everything works fine.
When saving the dataset on local disk (as opposed to s3 bucket) with `num_proc > 1`, everything works fine.

Thank you for your help! :)

### Steps to reproduce the bug

I tried without any storage options:

```
from datasets import load_dataset

sandbox_ds = load_dataset("openai_humaneval")
sandbox_ds["test"].save_to_disk(
    "s3://bucket-name/test_multiprocessing_saving/",
    num_proc=4,
)
```


and with the specific s3fs storage options: 
```
from datasets import load_dataset
from s3fs import S3FileSystem

def get_s3fs():
    return S3FileSystem()

sandbox_ds = load_dataset("openai_humaneval")
sandbox_ds["test"].save_to_disk(
    "s3://bucket-name/test_multiprocessing_saving/",
    num_proc=4,
    storage_options=get_s3fs().storage_options, # also tried: storage_options=S3FileSystem().storage_options
)
```

I'm guessing I might use `storage_options` parameter wrongly, but I didn't find anything online that made it work.

**NB**: Behavior is the same when trying to save the whole `DatasetDict`.

### Expected behavior

Progress bar fills in and saving is carried out.

### Environment info

`datasets==2.18.0`