Support multi-label classfication

[faroit]

Both, the dataset and the downloader are designed for multi-class classfication tasks. That means that a single label is used to store the data in hierachical folder structure as used in torchvision.datasets.ImageFolder.

Give that we also want to support #9, it can makes sense to switch to the webdataset format where each file is accompanied by a single label (e.g. json) file: resulting in a flat folder without any hierarchy.

eg.

e39871fd9fd74f55.jpg
e39871fd9fd74f55.json
f18b91585c4d3f3e.jpg
f18b91585c4d3f3e.json
ede6e66b2fb59aab.jpg
ede6e66b2fb59aab.json

That would allow us to save all gbif metadata in that json which, in turn, enables more diverse tasks such as unsupervised learning.

Of course we can also off support for both?