[C2] Tiny changes to adagrad to make it slightly better. (#33727)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/33727
Some small changes to adagrad (tiny bit faster, though there is more interesting diff in the stack on this).
Test Plan: Part of the stack
Reviewed By: chocjy
Differential Revision: D20029499
fbshipit-source-id: 7f4fddb9288d7881ef54673b17a0e19ef10d64c0
