Add a new method SummaryWriter.flush() (#20607)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/20607
Add a new method SummaryWriter.flush() that iterates through all of the FileWriters and flushes them
Reviewed By: orionr
Differential Revision: D15380124
fbshipit-source-id: 1975f3f61c5ae3754552bfdb23f2cd78f687d19f
