Add/make default values consistent for classes

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I will include default values for parameters of classes in the doc. Similarly, I will ensure the default values are consistent with the code and with how they are written in the doc.

I will start with the DictionaryLearning class.

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The PR for the DictionaryLearning class is here: scikit-learn#15435

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This PR has been merged.

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I have created a new PR to include default values for the SparseCode class. It can be found here: scikit-learn#15600

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The previous PR has been merged and I have now opened a new one found at scikit-learn#15753 which works on the default values in the dbscan class.

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Here is another PR for the kmeans class scikit-learn#15754

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The PR for the dbscan class has been merged. I opened an issue in the master branch to get more people to help out with this task: scikit-learn#15761