Add repo_id to DatasetInfo
```python
from datasets import load_dataset

ds = load_dataset("lhoestq/demo1", split="train")
ds = ds.map(lambda x: {}, num_proc=2).filter(lambda x: True).remove_columns(["id"])
print(ds.repo_id)
# lhoestq/demo1
```

- repo_id is None when the dataset doesn't come from the Hub, e.g. from Dataset.from_dict
- repo_id is set to None when concatenating datasets with different repo ids

related to https://github.com/huggingface/datasets/issues/4129

TODO:
- [ ] discuss if it's ok for now
- [ ] tests