Add solarization

[bordesf]

An implementation of Solarization that follow RandomSolarize from torchvision.transforms.

Here is an example with a threshold of 1. Left: original image; Middle: FFCV Solarization; Right: Torchvision.transforms

By the way, will it be possible to add the test_data/pig.png into a ffcv/beton file that will be saved in this repo ? This would enable the writing of automatics tests to ensure consistency with torchvision.transforms.