Incorporate CLDataset Downloader to Avalanche Datasets

[hammoudhasan]

Motivation:

Setting up large-scale continual learning datasets can often be cumbersome and involve complicated processing steps.

Solution:

Incorporate solution suggested in https://github.com/hammoudhasan/CLDatasets to allow for quick and easy installation of three large scale continual learning datasets, namely, CLOC, CGLM and ImageNet2K.