Add `module.training` to docs (#40923)
Summary:
A lot of people ask https://discuss.pytorch.org/t/check-if-model-is-eval-or-train/9395/3
Pull Request resolved: https://github.com/pytorch/pytorch/pull/40923
Reviewed By: pbelevich
Differential Revision: D22358799
Pulled By: zou3519
fbshipit-source-id: b5465ffedb691fb4811e097c4dbd7bbc405be09c
