Issue: IsADirectoryError when trying to download C4

**TLDR**:

I fail to download C4 and see a stacktrace originating in `IsADirectoryError` as an explanation for failure.

How can the problem be fixed? 

**VERBOSE**:

I use Python version 3.7 and have the following dependencies listed in my project:

```
datasets==1.2.0
apache-beam==2.26.0
```

When running the following code, where `/data/huggingface/unpacked/` contains a single unzipped `wet.paths` file manually downloaded as per the instructions for C4:

```
from datasets import load_dataset

load_dataset("c4", "en", data_dir="/data/huggingface/unpacked", beam_runner='DirectRunner')
```

I get the following stacktrace:

```
/Users/fredriko/venv/misc/bin/python /Users/fredriko/source/misc/main.py
Downloading and preparing dataset c4/en (download: Unknown size, generated: Unknown size, post-processed: Unknown size, total: Unknown size) to /Users/fredriko/.cache/huggingface/datasets/c4/en/2.3.0/8304cf264cc42bdebcb13fca4b9cb36368a96f557d36f9dc969bebbe2568b283...
Traceback (most recent call last):
  File "/Users/fredriko/source/misc/main.py", line 3, in <module>
    load_dataset("c4", "en", data_dir="/data/huggingface/unpacked", beam_runner='DirectRunner')
  File "/Users/fredriko/venv/misc/lib/python3.7/site-packages/datasets/load.py", line 612, in load_dataset
    ignore_verifications=ignore_verifications,
  File "/Users/fredriko/venv/misc/lib/python3.7/site-packages/datasets/builder.py", line 527, in download_and_prepare
    dl_manager=dl_manager, verify_infos=verify_infos, **download_and_prepare_kwargs
  File "/Users/fredriko/venv/misc/lib/python3.7/site-packages/datasets/builder.py", line 1066, in _download_and_prepare
    pipeline=pipeline,
  File "/Users/fredriko/venv/misc/lib/python3.7/site-packages/datasets/builder.py", line 582, in _download_and_prepare
    split_generators = self._split_generators(dl_manager, **split_generators_kwargs)
  File "/Users/fredriko/.cache/huggingface/modules/datasets_modules/datasets/c4/8304cf264cc42bdebcb13fca4b9cb36368a96f557d36f9dc969bebbe2568b283/c4.py", line 190, in _split_generators
    file_paths = dl_manager.download_and_extract(files_to_download)
  File "/Users/fredriko/venv/misc/lib/python3.7/site-packages/datasets/utils/download_manager.py", line 258, in download_and_extract
    return self.extract(self.download(url_or_urls))
  File "/Users/fredriko/venv/misc/lib/python3.7/site-packages/datasets/utils/download_manager.py", line 189, in download
    self._record_sizes_checksums(url_or_urls, downloaded_path_or_paths)
  File "/Users/fredriko/venv/misc/lib/python3.7/site-packages/datasets/utils/download_manager.py", line 117, in _record_sizes_checksums
    self._recorded_sizes_checksums[str(url)] = get_size_checksum_dict(path)
  File "/Users/fredriko/venv/misc/lib/python3.7/site-packages/datasets/utils/info_utils.py", line 80, in get_size_checksum_dict
    with open(path, "rb") as f:
IsADirectoryError: [Errno 21] Is a directory: '/'

Process finished with exit code 1
```

Comments:
I haven't tested C4 on my side so there so there may be a few bugs in the code/adjustments to make.
Here it looks like in c4.py, line 190 one of the `files_to_download` is `'/'` which is invalid.
Valid files are paths to local files or URLs to remote files.

Fixed once processed data is used instead:
-  #2575