Hub improvements (#26723)
Summary:
Resubmit of https://github.com/pytorch/pytorch/pull/25980.
Our old serialization was in tar (like `resnet18-5c106cde.pth` was in this format) so let's only support automatically unzip if checkpoints are zipfiles.
We can still manage to get it work with tarfile, but let's delay it when there's an ask.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/26723
Differential Revision: D17551795
Pulled By: ailzhang
fbshipit-source-id: 00b4e7621f1e753ca9aa07b1fe356278c6693a1e
