OSError with a custom dataset loading script
Hi,

Did some investigation.

To fix the dataset script on the Hub, append the following labels to the `names` list of the `upos_tags` field:
```'INTJ NOUN', 'AUX PRON', 'PART ADV', 'PRON ADP', 'INTJ INTJ', 'VERB NOUN', 'NOUN AUX'```.

This step is required to avoid an error due to missing labels in the following step which is:
```python
load_dataset("classla/janes_tag", split="validation", download_mode="force_redownload")
```
This will generate and cache the dataset, so specifying `download_mode` will not be required anymore unless you update the script/data on the Hub.
## Describe the bug
I am getting an OS error when trying to load the newly uploaded dataset classla/janes_tag. What puzzles me is that I have already uploaded a very similar dataset - classla/reldi_hr - with no issues. The loading scripts for the two datasets are almost identical and they have the same directory structure, yet I am only getting an error with janes_tag.

## Steps to reproduce the bug
```python
dataset = datasets.load_dataset('classla/janes_tag', split='validation')
```

## Expected results
Dataset correctly loaded.

## Actual results

Traceback (most recent call last):
  File "C:/mypath/test.py", line 91, in <module>
    load_and_print('janes_tag')
  File "C:/mypath/test.py", line 32, in load_and_print
    dataset = datasets.load_dataset('classla/{}'.format(ds_name), split='validation')
  File "C:\mypath\venv\lib\site-packages\datasets\load.py", line 1632, in load_dataset
    use_auth_token=use_auth_token,
  File "C:\mypath\venv\lib\site-packages\datasets\builder.py", line 608, in download_and_prepare
    dl_manager=dl_manager, verify_infos=verify_infos, **download_and_prepare_kwargs
  File "C:\mypath\venv\lib\site-packages\datasets\builder.py", line 704, in _download_and_prepare
    ) from None
OSError: Cannot find data file. 
Original error:
[Errno 2] No such file or directory: 'C:\\mypath\\.cache\\huggingface\\datasets\\downloads\\2c9996e44bdc5af9c89bffb9e6d7a3e42fdb2f56bacab45de13b20f3032ea7ca\\data\\train_all.conllup'

## Environment info
<!-- You can run the command `datasets-cli env` and copy-and-paste its output below. -->
- `datasets` version: 1.14.0
- Platform: Windows-10-10.0.19041-SP0
- Python version: 3.7.5
- PyArrow version: 3.0.0

OSError with a custom dataset loading script 
 ## Describe the bug
I am getting an OS error when trying to load the newly uploaded dataset classla/janes_tag. What puzzles me is that I have already uploaded a very similar dataset - classla/reldi_hr - with no issues. The loading scripts for the two datasets are almost identical and they have the same directory structure, yet I am only getting an error with janes_tag.

## Steps to reproduce the bug
```python
dataset = datasets.load_dataset('classla/janes_tag', split='validation')
```

## Expected results
Dataset correctly loaded.

## Actual results

Traceback (most recent call last):
  File "C:/mypath/test.py", line 91, in <module>
    load_and_print('janes_tag')
  File "C:/mypath/test.py", line 32, in load_and_print
    dataset = datasets.load_dataset('classla/{}'.format(ds_name), split='validation')
  File "C:\mypath\venv\lib\site-packages\datasets\load.py", line 1632, in load_dataset
    use_auth_token=use_auth_token,
  File "C:\mypath\venv\lib\site-packages\datasets\builder.py", line 608, in download_and_prepare
    dl_manager=dl_manager, verify_infos=verify_infos, **download_and_prepare_kwargs
  File "C:\mypath\venv\lib\site-packages\datasets\builder.py", line 704, in _download_and_prepare
    ) from None
OSError: Cannot find data file. 
Original error:
[Errno 2] No such file or directory: 'C:\\mypath\\.cache\\huggingface\\datasets\\downloads\\2c9996e44bdc5af9c89bffb9e6d7a3e42fdb2f56bacab45de13b20f3032ea7ca\\data\\train_all.conllup'

## Environment info
<!-- You can run the command `datasets-cli env` and copy-and-paste its output below. -->
- `datasets` version: 1.14.0
- Platform: Windows-10-10.0.19041-SP0
- Python version: 3.7.5
- PyArrow version: 3.0.0
 
 Hi,

Did some investigation.

To fix the dataset script on the Hub, append the following labels to the `names` list of the `upos_tags` field:
```'INTJ NOUN', 'AUX PRON', 'PART ADV', 'PRON ADP', 'INTJ INTJ', 'VERB NOUN', 'NOUN AUX'```.

This step is required to avoid an error due to missing labels in the following step which is:
```python
load_dataset("classla/janes_tag", split="validation", download_mode="force_redownload")
```
This will generate and cache the dataset, so specifying `download_mode` will not be required anymore unless you update the script/data on the Hub.