Force data files extraction if download_mode='force_redownload'
Avoids weird issues when redownloading a dataset due to cached data not being fully updated.

With this change, issues #3122 and https://github.com/huggingface/datasets/issues/2956 (not a fix, but a workaround) can be fixed as follows:
```python
dset = load_dataset(..., download_mode="force_redownload")
```