[BE]Clarify how to check memory saving if using gradient_as_bucket_view (#71483)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/71483
claify that peak memory saving should be checked after first iteration when using gradient_as_bucket_view
ghstack-source-id: 147271113
Test Plan: unit test
Reviewed By: rohan-varma
Differential Revision: D33662424
fbshipit-source-id: f760da38e166ae85234e526ddf1526269ea25d42
(cherry picked from commit a40dda20daa2fe051fcaa8fee5f3641aeea1da1c)
