Add docs and test guaranteeing indices from torch.nonzero ordered C-style (#19539)
Summary:
See #17556.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19539
Differential Revision: D15030151
Pulled By: ezyang
fbshipit-source-id: d46ee56a66d89b0113f86e3f8693dc1680d0adb9
