Unit 3: load oxford-pets dataset

[KuRRe8]

I tried running all notebooks of Unit 3, two of which require loading dataset oxford-pets.

The behavior of this dataset seems to be different from other datasets, as its internal 'image' key returns a dict containing bytes objects instead of image PIL objects, it needs to be explicitly converted to PIL. Otherwise, the subsequent code will report an error.

If approved, I will submit my pull requests