Added docs for context method mixins. Fixes issue #27365 (#28643)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/27365 .
This PR:
1. Makes Context method docs available.
2. Links [Extending torch autograd](https://pytorch.org/docs/stable/notes/extending.html#extending-torch-autograd) notes to Context method docs.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28643
Differential Revision: D18170089
Pulled By: albanD
fbshipit-source-id: a1119ea8e2f8a71f0d1aadf416f2f98343aa9b7b
