Dataset bugfixes

Highlights

This minor release bumps the pinned PyTorch version to v1.8.1, and brings a few bugfixes for datasets, including MNIST download not being available.

Bugfixes

• fix VOC datasets for 2007 (#3572)
• Update EMNIST url (#3567)
• Fix redirect behavior of datasets.utils.download_url (#3564)
• Fix MNIST download for minor release (#3559)