[easy] Fix docs for sd calculation in BatchNorm1d/3d for consistency with BatchNorm2d (#107308)
Fixes https://github.com/pytorch/pytorch/issues/100048
BatchNorm2d docs were updated in https://github.com/pytorch/pytorch/pull/97974. There have been a number of issues filed due to confusion about this so I think we should fix before branch cut
Pull Request resolved: https://github.com/pytorch/pytorch/pull/107308
Approved by: https://github.com/albanD
