Just came across this thread while looking for a solution myself.
(This may not be particularly useful since it has been a long time since your question.)
Here’s an example of how one could use untar_data if the dataset comes from an external source. Here we use the download_data helper function from fastai to download the flowers dataset.
url = 'http://download.tensorflow.org/example_images/flower_photos.tgz'
path = download_data(url)
path.as_posix()
# '/home/gg/.fastai/archive/flower_photos.tgz'
data = untar_data(path.as_posix()) # or pass str(path)
data
# Path('/home/gg/.fastai/data/flower_photos')