Dataset Viewer issue for openclimatefix/nimrod-uk-1km
If I understand correctly, this is due to the key `split` missing in the line https://huggingface.co/datasets/openclimatefix/nimrod-uk-1km/blob/main/nimrod-uk-1km.py#L41 of the script.
Maybe @albertvillanova could confirm. I'm having a look. Indeed there are several issues in this dataset loading script.

The one pointed out by @severo: for the default configuration "crops": https://huggingface.co/datasets/openclimatefix/nimrod-uk-1km/blob/main/nimrod-uk-1km.py#L244
- The download manager downloads `_URL`
- But `_URL` is not defined:  https://huggingface.co/datasets/openclimatefix/nimrod-uk-1km/blob/main/nimrod-uk-1km.py#L41
  ```python
  _URL = {'train': []}
  ```
- Afterwards, for each split, a different key in `_ULR` is used, but it only contains one key: "train"
  - "valid" key: https://huggingface.co/datasets/openclimatefix/nimrod-uk-1km/blob/main/nimrod-uk-1km.py#L260
  - "test key: https://huggingface.co/datasets/openclimatefix/nimrod-uk-1km/blob/main/nimrod-uk-1km.py#L269
  
These keys do not exist inside `_URL`, thus the error message reported in the viewer: 
```
Exception:     KeyError
Message:       'valid'
``` Would anyone want to submit a Hub PR (or open a Discussion for the authors to be aware) to this dataset? https://huggingface.co/datasets/openclimatefix/nimrod-uk-1km Hi, I'm the main author for that dataset, so I'll work on updating it! I was working on debugging some stuff awhile ago, which is what broke it.  I've opened a Discussion page, so that we can ask/answer and propose fixes until the script works properly: https://huggingface.co/datasets/openclimatefix/nimrod-uk-1km/discussions/1

CC: @julien-c @jacobbieker 
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