[DDP] Fix some issues with code example in DDP docstring (#67883)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/67883
cc pietern mrshenli pritamdamania87 zhaojuanmao satgera rohan-varma gqchen aazzolini osalpekar jiayisuse SciPioneer H-Huang
Test Plan: Imported from OSS
Reviewed By: zhaojuanmao
Differential Revision: D32190946
Pulled By: jamesr66a
fbshipit-source-id: a376324b95cbe833ffa606ecdfc6156432880f70
