datasets.map(f, num_proc=N) hangs with N>1 when run on import

### Describe the bug

If you `import` a module that runs `datasets.map(f, num_proc=N)` at the top-level, Python hangs.

### Steps to reproduce the bug

1. Create a file that runs datasets.map at the top-level:

```bash
cat < import_me.py
import datasets

the_dataset = datasets.load_dataset("openai/openai_humaneval")
the_dataset = the_dataset.map(lambda item: item, num_proc=2)
EOF
```

2. Start Python REPL:

```bash
uv run --python 3.12.3 --with "datasets==4.0.0" python3
Python 3.12.3 (main, Aug 14 2025, 17:47:21) [GCC 13.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
```

3. Import the file:

```python
import import_me
````

Observe hang.

### Expected behavior

Ideally would not hang, or would fallback to num_proc=1 with a warning.

### Environment info

- `datasets` version: 4.0.0
- Platform: Linux-6.14.0-29-generic-x86_64-with-glibc2.39
- Python version: 3.12.3
- `huggingface_hub` version: 0.34.4
- PyArrow version: 21.0.0
- Pandas version: 2.3.2
- `fsspec` version: 2025.3.0