Return the path of the scikit-learn data dir.
This folder is used by some large dataset loaders to avoid downloading the data several times.
By default the data dir is set to a folder named ‘scikit_learn_data’ in the user home folder.
Alternatively, it can be set by the ‘SCIKIT_LEARN_DATA’ environment variable or programmatically by giving an explicit folder path. The ‘~’ symbol is expanded to the user home folder.
If the folder does not already exist, it is automatically created.