Make AutogradMeta a private struct in Variable. (#27654)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/27654
Signed-off-by: Edward Z. Yang <ezyang@fb.com>
Test Plan: Imported from OSS
Differential Revision: D17886547
Pulled By: ezyang
fbshipit-source-id: ea0c5b40a5f34bc37657ed5d3bce9140063ddcbb
