新分类数据集: `imagenet10`, `imagenet100`, `imagenet1000`

All, FYI I've created 3 new classification datasets for use with debugging/tests/benchmarks: imagenet10, imagenet100, imagenet1000.

These are super small versions of imagenet that train/val in seconds with only 1 image per class (with all 1000 classes, only 100 classes, and only 10 classes). They are reduced in size to imgsz=160 and compressed with PIL so that i.e. imagenet10 is only 70kB, imagenet1000 is only 7MB.