Issue: NonMatchingChecksumError when checksum is None

I ran into the following error when adding a new dataset:

```bash
expected_checksums = {'https://adversarialglue.github.io/dataset/dev.zip': {'checksum': None, 'num_bytes': 40662}}
recorded_checksums = {'https://adversarialglue.github.io/dataset/dev.zip': {'checksum': 'efb4cbd3aa4a87bfaffc310ae951981cc0a36c6c71c6425dd74e5b55f2f325c9', 'num_bytes': 40662}}
verification_name = 'dataset source files'

    def verify_checksums(expected_checksums: Optional[dict], recorded_checksums: dict, verification_name=None):
        if expected_checksums is None:
            logger.info("Unable to verify checksums.")
            return
        if len(set(expected_checksums) - set(recorded_checksums)) > 0:
            raise ExpectedMoreDownloadedFiles(str(set(expected_checksums) - set(recorded_checksums)))
        if len(set(recorded_checksums) - set(expected_checksums)) > 0:
            raise UnexpectedDownloadedFile(str(set(recorded_checksums) - set(expected_checksums)))
        bad_urls = [url for url in expected_checksums if expected_checksums[url] != recorded_checksums[url]]
        for_verification_name = " for " + verification_name if verification_name is not None else ""
        if len(bad_urls) > 0:
            error_msg = "Checksums didn't match" + for_verification_name + ":\n"
>           raise NonMatchingChecksumError(error_msg + str(bad_urls))
E           datasets.utils.info_utils.NonMatchingChecksumError: Checksums didn't match for dataset source files:
E           ['https://adversarialglue.github.io/dataset/dev.zip']

src/datasets/utils/info_utils.py:40: NonMatchingChecksumError
```

## Expected results
The dataset downloads correctly, and there is no error.

## Actual results
Datasets library is looking for a checksum of None, and it gets a non-None checksum, and throws an error. This is clearly a bug.

Comments:
Hi @jxmorris12, thanks for reporting.

The objective of `verify_checksums` is to check that both checksums are equal. Therefore if one is None and the other is non-None, they are not equal, and the function accordingly raises a NonMatchingChecksumError. That behavior is expected.

The question is: how did you generate the expected checksum? Normally, it should not be None. To properly generate it (it is contained in the `dataset_infos.json` file), you should have runned: https://github.com/huggingface/datasets/blob/master/ADD_NEW_DATASET.md
```shell
datasets-cli test <your-dataset-folder> --save_infos --all_configs
```

On the other hand, you should take into account that the generation of this file is NOT mandatory for personal/community datasets (we only require it for "canonical" datasets, i.e., datasets added to our library GitHub repository: https://github.com/huggingface/datasets/tree/master/datasets). Therefore, other option would be just to delete the `dataset_infos.json` file. If that file is not present, the function `verify_checksums` is not executed.

Finally, you can circumvent the `verify_checksums` function by passing `ignore_verifications=True` to `load_dataset`:
```python
load_dataset(..., ignore_verifications=True)
``` 

Thanks @albertvillanova!

That's fine. I did run that command when I was adding a new dataset. Maybe because the command crashed in the middle, the checksum wasn't stored properly. I don't know where the bug is happening. But either (i) `verify_checksums` should properly handle this edge case, where the passed checksum is None or (ii) the `datasets-cli test` shouldn't generate a corrupted dataset_infos.json file.

Just a more high-level thing, I was trying to follow the instructions for adding a dataset in the CONTRIBUTING.md, so if running that command isn't even necessary, that should probably be mentioned in the document, right? But that's somewhat of a moot point, since something isn't working quite right internally if I was able to get into this corrupted state in the first place, just by following those instructions.

Hi @jxmorris12,

Definitely, your `dataset_infos.json` was corrupted (and wrongly contains expected None checksum). 

While we further investigate how this can happen and fix it, feel free to delete your `dataset_infos.json` file and recreate it with:
```shell
datasets-cli test <your-dataset-folder> --save_infos --all_configs
```

Also note that `verify_checksum` is working as expected: if it receives a None and and a non-None checksums as input pair, it must raise an exception: they are not equal. That is not a bug.

At a higher level, also note that we are preparing the release of `datasets` version 2.0, and some docs are being updated...

In order to add a dataset, I think the most updated instructions are in our official documentation pages: https://huggingface.co/docs/datasets/share

Thanks for the info. Maybe you can update the contributing.md if it's not up-to-date.

Hi @jxmorris12, we have discovered the bug why `None` checksums wrongly appeared when generating the `dataset_infos.json` file:
- #3892

The fix will be accessible once this PR merged. And we are planning to do our 2.0 release today.

We are also working on updating all our docs for our release today.

Thanks @albertvillanova - congrats on the release!