[Feature Request] Optionally save ImageFolder information

Currently ImageFolder indexes a directory when the object is created. For some datasets (e.g., Places365, ImageNet, etc.) this can take several minutes. In the case of ImageNet-22K, this can take over 30 minutes on an SSD (albeit on an NFS system). Torch saved this information, but PyTorch requires re-indexing every run. I request the option to save and restore the indexing information.