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
