Small clarification to amp gradient penalty example (#44667)
Summary:
requested by https://discuss.pytorch.org/t/what-is-the-correct-way-of-computing-a-grad-penalty-using-amp/95827/3
Pull Request resolved: https://github.com/pytorch/pytorch/pull/44667
Reviewed By: mruberry
Differential Revision: D23692768
Pulled By: ngimel
fbshipit-source-id: 83c61b94e79ef9f86abed2cc066f188dce0c8456
