Loading of datasets changed in #3110 returns no examples 
Ok I found the issue. It's not related to python 3.8 in itself though. This issue happens because your local installation of `datasets` is outdated compared to the changes to datasets in #3110

To fix this you just have to pull the latest changes from `master` :)

Let me know if that helps !

--------------

Here are more details about my investigation:

It's possible to reproduce this issue if you use `datasets<=1.15.1` or before b6469baa22c174b3906c631802a7016fedea6780 and if you load the dataset after revision b6469baa22c174b3906c631802a7016fedea6780. This is because `dl_manager.iter_archive` had issues at that time (and it was not used anywhere anyway).

In particular it was returning the absolute path to extracted files instead of the relative path of the file inside the archive. This was an issue because `dl_manager.iter_archive` isn't supposed to extract the TAR archive. Instead, it iterates over all the files inside the archive, without creating a directory with the extracted content.

Therefore if you want to use the datasets on `master`, make sure that you have an up-to-date local installation of `datasets` as well, or you may face incompatibilities like this.
## 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
 
 Ok I found the issue. It's not related to python 3.8 in itself though. This issue happens because your local installation of `datasets` is outdated compared to the changes to datasets in #3110

To fix this you just have to pull the latest changes from `master` :)

Let me know if that helps !

--------------

Here are more details about my investigation:

It's possible to reproduce this issue if you use `datasets<=1.15.1` or before b6469baa22c174b3906c631802a7016fedea6780 and if you load the dataset after revision b6469baa22c174b3906c631802a7016fedea6780. This is because `dl_manager.iter_archive` had issues at that time (and it was not used anywhere anyway).

In particular it was returning the absolute path to extracted files instead of the relative path of the file inside the archive. This was an issue because `dl_manager.iter_archive` isn't supposed to extract the TAR archive. Instead, it iterates over all the files inside the archive, without creating a directory with the extracted content.

Therefore if you want to use the datasets on `master`, make sure that you have an up-to-date local installation of `datasets` as well, or you may face incompatibilities like this.