fix comments in gradcheck (#38877)
Summary:
Follow up to https://github.com/pytorch/pytorch/issues/38774
Pull Request resolved: https://github.com/pytorch/pytorch/pull/38877
Differential Revision: D21697680
Pulled By: albanD
fbshipit-source-id: f7cf6fb79f56eac2afceec7167c26e25f20a665d
