Outdated overview tutorial

[leon-costa]

Describe the bug
The Overview tutorial doesn't work with the current code.

To Reproduce
Follow the steps of the tutorial.

Screenshots

Traceback (most recent call last):
  File "/home/leon/projects/solo-learn/tutorial-unfixed.py", line 13, in <module>
    from solo.utils.classification_dataloader import prepare_data as prepare_data_classification
ModuleNotFoundError: No module named 'solo.utils.classification_dataloader'

Versions
The main branch of solo-learn.

Additional comments
I started to fix this in leon-costa@5f3ff8f but I didn't go further than fixing the imports. The rest of the code probably needs to be updated as well.