Move loading out of preprocessing

[juliabeliaeva]

It seems that loading functionality should be moved out of preprocessing to the Dataset implementations.

1. Loading images is orthogonal to preprocessing. Having a loading stage limits preprocessors functionality, as it prohibits sharing preprocessors between datasets or applying them to images loaded from elsewhere.
2. Knowledge about the source of the images is intrinsic to some of the Dataset implementations, specifically OnFlyImageDataset.
3. The code creating dataset implementations already directly accesses loading stage of the provided Preprocessing instance.