Fix DDP documentation (#46861)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/46861
Noticed that in the DDP documentation:
https://pytorch.org/docs/master/generated/torch.nn.parallel.DistributedDataParallel.html?highlight=distributeddataparallel
there were some examples with `torch.nn.DistributedDataParallel`, fix this to
read `torch.nn.parallel.DistributedDataParallel`.
ghstack-source-id: 115453703
Test Plan: ci
Reviewed By: pritamdamania87, SciPioneer
Differential Revision: D24534486
fbshipit-source-id: 64b92dc8a55136c23313f7926251fe825a2cb7d5
