Add docs describing saved tensor hooks (#62362)

Add docs describing saved tensor hooks (#62362)

Summary:
Add section to the Autograd mechanics docs to describe the recently
exposed saved tensors (https://github.com/pytorch/pytorch/issues/52451), how to register packing / unpacking
hooks (https://github.com/pytorch/pytorch/issues/60975) and how to use default hooks (https://github.com/pytorch/pytorch/issues/61834)

Sister PR: https://github.com/pytorch/pytorch/issues/62361 (will add a link from autograd.rst to notes/autograd in whatever PR does not land first)

Pull Request resolved: https://github.com/pytorch/pytorch/pull/62362

Reviewed By: soulitzer

Differential Revision: D30453177

Pulled By: Varal7

fbshipit-source-id: f5759977b069ff0ef36a47b08856d297691a6caa