use `const auto&` as type for grad alias (#63949)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/63949
This is an extension of the discussion in
https://github.com/pytorch/pytorch/pull/63040#discussion_r687793027.
Test Plan: Imported from OSS
Reviewed By: albanD
Differential Revision: D30546789
Pulled By: dagitses
fbshipit-source-id: 3046aff4f129d5492d73dfb67717a824e16ffee8
