Add comment explaining purpose of the accumulate_grad argument (#47266)
Summary:
Addressing a comment from a PR that has already been merged https://github.com/pytorch/pytorch/issues/46855
https://github.com/pytorch/pytorch/pull/46855#discussion_r515161953
Pull Request resolved: https://github.com/pytorch/pytorch/pull/47266
Reviewed By: agolynski
Differential Revision: D24709017
Pulled By: soulitzer
fbshipit-source-id: 3c104c2fef90ffd75951ecef4ae9e938d4b12d8c
