Datasets 4.0 map function causing column not found
Hi, I tried to reproduce this issue on the latest `main` branch but it seems to be working correctly now. My test script (which creates a dummy dataset and applies the `.map()` function) successfully creates and accesses the new column without a `KeyError`.

It's possible this was fixed by a recent commit. The maintainers might want to consider closing this issue. Hi, have you tried on a large dataset (200GB+) perhaps? I will try my best to do a rerun with main branch when I have the time. I ran it on a small dataset, maybe that’s why I didn’t hit the issue. If it still shows up on your side with the latest main, let me know. I can try it on a bigger set too.
### Describe the bug

Column returned after mapping is not found in new instance of the dataset.

### Steps to reproduce the bug

Code for reproduction. After running get_total_audio_length, it is errored out due to `data` not having `duration`
```
def compute_duration(x):
    return {"duration": len(x["audio"]["array"]) / x["audio"]["sampling_rate"]}


def get_total_audio_length(dataset):
    data = dataset.map(compute_duration, num_proc=NUM_PROC)
    print(data)
    durations=data["duration"]
    total_seconds = sum(durations)
    return total_seconds
```

### Expected behavior

New datasets.Dataset instance should have new columns attached.

### Environment info

- `datasets` version: 4.0.0
- Platform: Linux-5.4.0-124-generic-x86_64-with-glibc2.31
- Python version: 3.10.13
- `huggingface_hub` version: 0.33.2
- PyArrow version: 20.0.0
- Pandas version: 2.3.0
- `fsspec` version: 2023.12.2