Imagenet num_img_per_val_class isn't part of ffcv file hash

[lebrice]

When creating the train_ffcv file, I take into account all the ffcv-related parameters to create a hash that gets put in the file name. However, this num_img_per_val_class isn't in there, so even if it gets changed, the same ffcv file will be used for the train dataloader, and the length of the dataloader will not change! (note: the length of the val dataloader will change).