[CUDA graphs] moves memory sharing intro paragraph (#64996)
Summary:
Puts memory sharing intro under Sharing memory... header, where it should have been all along.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/64996
Reviewed By: mruberry
Differential Revision: D30948619
Pulled By: ngimel
fbshipit-source-id: 5d9dd267b34e9d3fc499d4738377b58a22da1dc2
