Harry, you need to get the data “ready” for training so it needs some basic transforms. If you use this you don’t get data augmentation. sz x sz x 3 is the size of the images that you will be training on.
tfms = tfms_from_model(f_model, sz)
Here is an example on why you need basic transformations. You may have images of different sizes but you need training batches to a consistent size. In order to get that your images need to have all the same size. In this library they all get transform into a square. The library has different ways of doing that.