fix comment index_size->output_size (#29831)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/29831
As title. Thanks Aleks Zi for finding this!
Test Plan: Just changing comments
Reviewed By: zlateski
Differential Revision: D18511259
fbshipit-source-id: 5f1ad9ba53db9b22622a556ec214ced361ec016a
