Remove unused six code for Python 2/3 compatibility (#48077)
Summary:
This is basically a reborn version of https://github.com/pytorch/pytorch/issues/45254 .
Ref: https://github.com/pytorch/pytorch/issues/42919
Pull Request resolved: https://github.com/pytorch/pytorch/pull/48077
Reviewed By: ngimel
Differential Revision: D25687042
Pulled By: bugra
fbshipit-source-id: 05f20a6f3c5212f73d0b1505b493b720e6cf74e5
