[CUDA graphs] Prototype API and documentation (#63269)
Summary:
RFC: https://github.com/pytorch/pytorch/issues/61880
Pull Request resolved: https://github.com/pytorch/pytorch/pull/63269
Reviewed By: mruberry
Differential Revision: D30596643
Pulled By: ngimel
fbshipit-source-id: b1f8061406364b667e2c2d4d30fbce1f0d8456be
