Add automatic download for imagenet list files.

[xperzy]

Describe your feature request
Now the dataset for ImageNet2012 requires the "train_list.txt" and "val_list.txt" in the same folder as "train" and "val" which stores the images.

Users are required to download the txt files and locate to the correct dir manually, which can be automated by our code.

Describe the possible solution

1. txt files can be uploaded in github repo
2. Add check and download code in Imagenet Dataset class