[BC-breaking] remove unnecessary split argument from datasets

[parmeet]

This PR remove the split argument from CC100 and Enwik9 datasets. This is superficial just to comply with other datasets format. Specially datasets that serves unsupervised learning, it is often the case that it is just a collection of text. The dev/test split are not always necessarily available.

BC-Breaking: This PR is BC-breaking for EnWiK9 dataset as we are removing the split argument.

[Nayef211]

Overall the changes look good to me. A couple of follow-ups:

• Let's remove test_enwik9_split_argument from the corresponding test for the EnWik9 dataset
• Should we make note of the BC breaking changes (removing the split arg from EnWik9) in our release notes?

[Nayef211]

Any reason we're removing the _add_docstring_header decorator and manually adding the docstrings? Can we modify the _add_docstring_header to work with a dataset function that doesn't have a split arg?

[parmeet]

The decorator is doing a job of avoiding writing doc-string and make strong assumptions about datasets. But it's scope is fairly limited (for eg: it does not handle cases when there are no splits or when we have additional arguments other than root and split). Overall, I think we probably need to get rid of _add_doctring_header decorator in light of this issue #1588 anyway since we plan to expand the scope of documentation of what we currently have.

[Nayef211]

I think we still need to remove the split argument when calling the EnWik9 dataset as well as the @parameterized.expand(["train"]) decorator for the test_enwik9 method to ensure all unit tests are passing.

On the bright side, it looks like our mocked tests are already providing useful signals when we make BC breaking changes to our datasets 😁

[parmeet]

I think we still need to remove the split argument when calling the EnWik9 dataset as well as the @parameterized.expand(["train"]) decorator for the test_enwik9 method to ensure all unit tests are passing.

On the bright side, it looks like our mocked tests are already providing useful signals when we make BC breaking changes to our datasets 😁

haha, true that! :)

[Nayef211]

LGTM! Let's fix the stylecheck errors before merging