Make set_grad_accumulator private (friend class SavedVariable) (#27666)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/27666
Signed-off-by: Edward Z. Yang <ezyang@fb.com>
Test Plan: Imported from OSS
Differential Revision: D17886544
Pulled By: ezyang
fbshipit-source-id: b9ff845cb1e5ec6f7cb4f2fa171403d555014248
