!pip uninstall torch torchvision -y
!pip install torch==1.4.0 torchvision==0.5.0
Put these two line at the top of the notebook and run it. Restart the runtime and you are good to go. torch updated the release version on 21 april and fastai v1.0.60 became incompatable. although fastai has fixed the issue to make it compatable but unless someboday has installed fastai from source itself, getting the fixed changes is difficult. fastai need to create another relaese with updated changes.