Small clarification of torch.cuda.amp multi-model example (#41203)
Summary:
some people have been confused by `retain_graph` in the snippet, they thought it was an additional requirement imposed by amp.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/41203
Differential Revision: D22463700
Pulled By: ngimel
fbshipit-source-id: e6fc8871be2bf0ecc1794b1c6f5ea99af922bf7e
