Fix a problem when removing parametrizations (#55456)
Summary:
There was an error when removing a parametrization with `leave_parametrized=True`. It had escaped the previous tests. This PR should fix that.
**Edit.**
I also took this chance to fix a few mistakes that the documentation had, and to also write the `set_original_` in a more compact way.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/55456
Reviewed By: mrshenli
Differential Revision: D27620481
Pulled By: albanD
fbshipit-source-id: f1298ddbcf24566ef48850c62a1eb4d8a3576152
