Add optional normalization to fetch_20newsgroups_vectorized

[stephantul]

Reference Issues/PRs

Fixes #14738

What does this implement/fix? Explain your changes.

fetch_20newsgroups_vectorized normalizes the data returned by CountVectorizer without indicating this to the user. I added an argument to the function which allows users to switch off normalization (normalization is still performed by default), as well as a comment in the docstring indicating that normalization is performed.

Any other comments?

Because the argument to the function is named normalize (as suggested by @rth) , I imported the function sklearn.preprocessing.normalize as normalize_func to clearly distinguish them. I'm not sure if this is correct.

[glemaitre]

In addition, you need to add an entry in doc/whats_new/v0.22.rst, in the according section. You'll need to mention that the parameter was added.

[stephantul]

Thanks for your comments, I added the changes.

[glemaitre]

Last thing. We need a test in datasets/tests/test_20news.py which should check that normalize is behaving properly. So we could check that if normalize=True, we get the unit norm (we only need to check it for a couple of samples), and otherwise this is not.

[rth]

It would be good to add a test to sklearn/datasets/tests/test_20news.py than loading it with normalize=True is equivalent to applying normalize after loading with normalize=False (if the run time is not too long). Otherwise LGTM.

[stephantul]

Thanks for your replies, I made a test using the first 100 items that checks for equivalence and the norm.

[stephantul]

I think I addressed all points. Sorry for letting this hang for a bit, I assumed you were busy.

[rth]

LGTM, thanks @stephantul !

I can confirm that test_20news_normalization runs fine locally, not sure what is going on with codecov CI.

[jnothman]

Merging in the latest master often fixes codecov

[stephantul]

Hey, I merged master into my branch, but codecov still fails 😞. Let me know if I need to do other things to make this work.

[jnothman]

Thanks