Reference amp tutorial (recipe) from core amp docs (#44725)
Summary:
https://pytorch.org/tutorials/recipes/recipes/amp_recipe.html is live. Core amp docs should reference it.
Also i fixed some typos in the `zero_grad` docs we ignored when git was behaving weirdly during ngimel 's merge of https://github.com/pytorch/pytorch/pull/44423.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/44725
Reviewed By: mruberry
Differential Revision: D23723807
Pulled By: ngimel
fbshipit-source-id: ca0b76365f8ca908bd978e3b38bf81857fa6c2a3
