Delete `raise_from` from `torch._six` (#43981)
Summary:
No need for compatibility wrapper in Python3+ world
Pull Request resolved: https://github.com/pytorch/pytorch/pull/43981
Reviewed By: seemethere
Differential Revision: D23458325
Pulled By: malfet
fbshipit-source-id: 00f822895625f4867c22376fe558c50316f5974d
