Fix default value of `num_replicas` in DistributedSampler docstring (#48135)
Summary:
Change default value of `num_replicas` from `rank` to `world_size` in DistributedSampler docstring.
Addresses https://github.com/pytorch/pytorch/issues/48055
Pull Request resolved: https://github.com/pytorch/pytorch/pull/48135
Reviewed By: gchanan
Differential Revision: D25045328
Pulled By: rohan-varma
fbshipit-source-id: 6f84f7bb69087d8dae931cda51891b3cb1894306
