DOC: Add missing args for index_add (#38213)
Summary:
Fix https://github.com/pytorch/pytorch/issues/37752 by updating `index_add`documentation as suggested by danpovey
Pull Request resolved: https://github.com/pytorch/pytorch/pull/38213
Reviewed By: ilia-cher
Differential Revision: D21506728
Pulled By: ngimel
fbshipit-source-id: 3c08bc3743cd4ba8c0c97b7d359d35e82f0127ac
