There’s a change in PyTorch 1.6 to the file format that is used for saving models (see Deprecations). In fastai v1, in v1.0.63 we’re now setting a flag to tell PyTorch to continue to save models in the same format as before, to avoid compatibility problems.
However, in fastai v2, it’ll save in the new PyTorch 1.6 format, since fastai v2 requires PyTorch 1.6 anyway.