Skip model download on import

[bogdan-galileo]

Importing PHash causes the import of the CNN class and triggers the download of the Mobilenet model from torchvision.

Although very small, it's just and extra log that can be distracting in applications.

Can I get rid of it ? This was just introduced in the PR #190

It seems to be caused by the import of the class CNN from imagededup.methods.cnn, which downloads the model as part as evaluating the default argument of __init__.