Update scikit-learn imports

[farhadmd7]

Hi,
I want to use your interesting approach in the package but the last scikit learn version which supported
from sklearn.neighbors import DistanceMetric is according to the docs officially 0.24. (from 2021-22)
DistanceMetric has to instead be imported from sklearn.metrics

With regards to reproducibility of notebooks:

after making these changes in this commit I could reproduce the Swissroll Example.ipynb notebook but not the Line Example.ipynb.

and got following error:

There because of old implementation of pygsp, in line causes the scipy to throw this line

as a workaround in this commit
I used sklrean to create a radius-based nearest neighbors graph that I believe matches what pygsp.graphs.NNGraph construct to avoid said issue

I wrapped the created graph with pygsp instance so it doesnt break the usage in the notebooks or in case someone using the dataset class

the produced outputs after these changes are very similar to what you have in the repository

if these changes aligens with what you meant to create with Line Dataset, it would greatly help me if you accept this PR.