Remove six dependency (#27282)
Summary:
https://github.com/pytorch/pytorch/pull/27136 added a dependency on `six`, which is not available by default and is not marked as a dependency on PyTorch binaries, causing torchvision CI to break, see https://circleci.com/gh/pytorch/vision/20778?utm_campaign=vcs-integration-link&utm_medium=referral&utm_source=github-build-link for example.
This PR use `torch._six` instead of `six` as a replacement.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/27282
Reviewed By: lerks
Differential Revision: D17737561
Pulled By: fmassa
fbshipit-source-id: 7dcd0cc2c8bab27b8f4535f664f60388818d3497
