Loading of datasets changed in #3110 returns no examples 
Hi @eladsegal, thanks for reporting.

I am sorry, but I can't reproduce the bug:
```
In [1]: from datasets import load_dataset

In [2]: ds = load_dataset("qasper")
Downloading: 5.11kB [00:00, ?B/s]
Downloading and preparing dataset qasper/qasper (download: 9.88 MiB, generated: 35.11 MiB, post-processed: Unknown size, total: 44.99 MiB) to .cache\qasper\qasper\0.1.0\b99154d2a15aa54bfc669f82b2eda715a2e342e81023d39613b0e2920fdb3ad8...
Dataset qasper downloaded and prepared to .cache\qasper\qasper\0.1.0\b99154d2a15aa54bfc669f82b2eda715a2e342e81023d39613b0e2920fdb3ad8. Subsequent calls will reuse this data.
100%|████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████| 2/2 [00:00<?, ?it/s]

In [3]: ds
Out[3]:
DatasetDict({
    train: Dataset({
        features: ['id', 'title', 'abstract', 'full_text', 'qas'],
        num_rows: 888
    })
    validation: Dataset({
        features: ['id', 'title', 'abstract', 'full_text', 'qas'],
        num_rows: 281
    })
})
``` 

This makes me suspect that the origin of the problem might be the cache: I didn't have this dataset in my cache, although I guess you already had it, before the code change introduced by #3110.

@lhoestq might it be possible that the code change introduced by #3110 makes "inaccessible" all previously cached TAR-based datasets?
- Before the caching system downloaded and extracted the tar dataset
- Now it only downloads the tar dataset (no extraction is done)
## Describe the bug
Loading of datasets changed in https://github.com/huggingface/datasets/pull/3110 returns no examples:
```python
DatasetDict({
    train: Dataset({
        features: ['id', 'title', 'abstract', 'full_text', 'qas'],
        num_rows: 0
    })
    validation: Dataset({
        features: ['id', 'title', 'abstract', 'full_text', 'qas'],
        num_rows: 0
    })
})
```

## Steps to reproduce the bug
Load any of the datasets that were changed in https://github.com/huggingface/datasets/pull/3110:
```python
from datasets import load_dataset
load_dataset("qasper")

# The problem only started with the commit of #3110
load_dataset("qasper", revision="b6469baa22c174b3906c631802a7016fedea6780")
```

## Expected results
```python
DatasetDict({
    train: Dataset({
        features: ['id', 'title', 'abstract', 'full_text', 'qas'],
        num_rows: 888
    })
    validation: Dataset({
        features: ['id', 'title', 'abstract', 'full_text', 'qas'],
        num_rows: 281
    })
})
```
Which can be received when specifying revision of the commit before https://github.com/huggingface/datasets/pull/3110:
```python
from datasets import load_dataset
load_dataset("qasper", revision="acfe2abda1ca79f0ce5c1896aa83b4b78af76b7d")
```

## Environment info
<!-- You can run the command `datasets-cli env` and copy-and-paste its output below. -->
- `datasets` version: 1.15.2.dev0 (master)
- Python version: 3.8.10
- PyArrow version: 3.0.0

Loading of datasets changed in #3110 returns no examples  
 ## Describe the bug
Loading of datasets changed in https://github.com/huggingface/datasets/pull/3110 returns no examples:
```python
DatasetDict({
    train: Dataset({
        features: ['id', 'title', 'abstract', 'full_text', 'qas'],
        num_rows: 0
    })
    validation: Dataset({
        features: ['id', 'title', 'abstract', 'full_text', 'qas'],
        num_rows: 0
    })
})
```

## Steps to reproduce the bug
Load any of the datasets that were changed in https://github.com/huggingface/datasets/pull/3110:
```python
from datasets import load_dataset
load_dataset("qasper")

# The problem only started with the commit of #3110
load_dataset("qasper", revision="b6469baa22c174b3906c631802a7016fedea6780")
```

## Expected results
```python
DatasetDict({
    train: Dataset({
        features: ['id', 'title', 'abstract', 'full_text', 'qas'],
        num_rows: 888
    })
    validation: Dataset({
        features: ['id', 'title', 'abstract', 'full_text', 'qas'],
        num_rows: 281
    })
})
```
Which can be received when specifying revision of the commit before https://github.com/huggingface/datasets/pull/3110:
```python
from datasets import load_dataset
load_dataset("qasper", revision="acfe2abda1ca79f0ce5c1896aa83b4b78af76b7d")
```

## Environment info
<!-- You can run the command `datasets-cli env` and copy-and-paste its output below. -->
- `datasets` version: 1.15.2.dev0 (master)
- Python version: 3.8.10
- PyArrow version: 3.0.0
 
 Hi @eladsegal, thanks for reporting.

I am sorry, but I can't reproduce the bug:
```
In [1]: from datasets import load_dataset

In [2]: ds = load_dataset("qasper")
Downloading: 5.11kB [00:00, ?B/s]
Downloading and preparing dataset qasper/qasper (download: 9.88 MiB, generated: 35.11 MiB, post-processed: Unknown size, total: 44.99 MiB) to .cache\qasper\qasper\0.1.0\b99154d2a15aa54bfc669f82b2eda715a2e342e81023d39613b0e2920fdb3ad8...
Dataset qasper downloaded and prepared to .cache\qasper\qasper\0.1.0\b99154d2a15aa54bfc669f82b2eda715a2e342e81023d39613b0e2920fdb3ad8. Subsequent calls will reuse this data.
100%|████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████| 2/2 [00:00<?, ?it/s]

In [3]: ds
Out[3]:
DatasetDict({
    train: Dataset({
        features: ['id', 'title', 'abstract', 'full_text', 'qas'],
        num_rows: 888
    })
    validation: Dataset({
        features: ['id', 'title', 'abstract', 'full_text', 'qas'],
        num_rows: 281
    })
})
``` 

This makes me suspect that the origin of the problem might be the cache: I didn't have this dataset in my cache, although I guess you already had it, before the code change introduced by #3110.

@lhoestq might it be possible that the code change introduced by #3110 makes "inaccessible" all previously cached TAR-based datasets?
- Before the caching system downloaded and extracted the tar dataset
- Now it only downloads the tar dataset (no extraction is done)