Add docstring for saved tensors default hooks (#62361)
Summary:
Add documentation for the saved tensors default hooks introduced in https://github.com/pytorch/pytorch/issues/61834 / https://github.com/pytorch/pytorch/issues/62563
Sister PR: https://github.com/pytorch/pytorch/issues/62362 (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/62361
Reviewed By: zou3519
Differential Revision: D30081997
Pulled By: Varal7
fbshipit-source-id: cb923e943e1d96db9669c1d863d693af30910c62
