Update links in DDP note (#47663)
Summary:
Update the links in https://pytorch.org/docs/stable/notes/ddp.html#.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/47663
Reviewed By: smessmer
Differential Revision: D24951684
Pulled By: ezyang
fbshipit-source-id: c1c104d76cf0292a7fc75a627bf76bb56fea72d0
