Write unit test coverage for SafeDataset and SafeDataLoader, along with the functions in utils.py.

Description

It would be nice if there is a copy button in the docs and readme examples to make it easier for users to try the examples and play with the code. This should be easy to implement, maybe using a sphinx library. The docs are in the docs folder in the repo, I will let this issue open for new comers ;)

It would be good to add some check in case there are non-image files in an image directory.

Describe the solution you'd like
A simple filter would suffice, e.g.

If not image file: 
    skip

Describe alternatives you've considered
A: Do nothing - potential for tool to fail while processing data, which could waste user's time
B: Filter at the DataFrame level - best not t