Backward function will set a flag if it released variables (#21533)
Summary:
This is a fix for https://github.com/pytorch/pytorch/issues/21469
Currently there is no way to define if backward function released variables when variables were added to a vector. This change will set a flag if function has saved variables and they were released. So we will prevent if somebody will call this function again with already released variables.
Functions that do not have saved variables can be called multiple times for BC
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21533
Differential Revision: D15810481
Pulled By: ifedan
fbshipit-source-id: 5663e0c14f1b65727abc0d078aef348078d6a543
