Allow empty class folders in ImageFolder

[NicolasHug]

Follow up to #4925 and as summarized in #4925 (comment), we need to allow empty class folders in ImageFolder i.e. we need a way for users to bypass this error:

vision/torchvision/datasets/folder.py

Lines 98 to 99 in c8c3839

	if empty_classes:
	msg = f"Found no valid file for the classes {', '.join(sorted(empty_classes))}. "

The most obvious way to enable that is by adding a new allow_empty parameter to make_dataset() and ImageFolder() / DatasetFolder(). (name TBD).