losing DatasetInfo in Dataset.map when num_proc > 1
Hi ! This issue comes from the fact that `map()` with `num_proc>1` shards the dataset in multiple chunks to be processed (one per process) and merges them. The DatasetInfos of each chunk are then merged together, but for some fields like `dataset_name` it's not been implemented and default to None.

The DatasetInfo merge is defined here, in case you'd like to contribute an improvement: 

https://github.com/huggingface/datasets/blob/d2e0034122a788015c0834a72e6c6279e7ecbac5/src/datasets/info.py#L269-L270 #self-assign
### Describe the bug

Hello and thanks for developing this package!

When I process a Dataset with the map function using multiple processors some set attributes of the DatasetInfo get lost and are None in the resulting Dataset.    

### Steps to reproduce the bug

```python
from datasets import Dataset, DatasetInfo


def run_map(num_proc):
    dataset = Dataset.from_dict(
        {"col1": [0, 1], "col2": [3, 4]},
        info=DatasetInfo(
            dataset_name="my_dataset",
        ),
    )

    ds = dataset.map(lambda x: x, num_proc=num_proc)
    print(ds.info.dataset_name)


run_map(1)
run_map(2)
```

This puts out:

```bash
Map: 100%|██████████| 2/2 [00:00<00:00, 724.66 examples/s]
my_dataset
Map (num_proc=2): 100%|██████████| 2/2 [00:00<00:00, 18.25 examples/s]
None
```

### Expected behavior

I expect the DatasetInfo to be kept as it was and there should be no difference in the output of running map with num_proc=1 and num_proc=2. 

Expected output:
```bash
Map: 100%|██████████| 2/2 [00:00<00:00, 724.66 examples/s]
my_dataset
Map (num_proc=2): 100%|██████████| 2/2 [00:00<00:00, 18.25 examples/s]
my_dataset
```

### Environment info

- `datasets` version: 2.16.1
- Platform: Linux-5.10.102.1-microsoft-standard-WSL2-x86_64-with-glibc2.17
- Python version: 3.8.18
- `huggingface_hub` version: 0.20.2
- PyArrow version: 12.0.1
- Pandas version: 2.0.3
- `fsspec` version: 2023.9.2