I had the same problem earlier. The solution is to update it to the latest version NOT using conda update -c fastai fastai suggested by others (it will update fastai to 1.0.6), but use the dev install option.
A simpler approach I used is to simply install using pip install git+https://github.com/fastai/fastai
Then when you run python -c "import fastai; print(fastai.__version__)" you should see 1.0.12.dev0
If you installed fastai using conda, for some reason it only installs v1.0.6 like the one you have, but that version is at least 4 days behind (if you take a look at your own your_conda_env_path/fastai/vision/models/__init__.py and compare it with the latest one). The one from 1.0.6 doesn’t import any torchvision models. I think it may have been due to Github’s recent server issue and it may be fixed by tomorrow (as of now it hasn’t been fixed).