Keep alive graph when creating iterators from it (#51951)
Summary:
Previously, the graph might have been delete while Python still has iterators, leading to segfaults.
This does not fully work for iterators from Nodes and Blocks as they may be invalidated when the owning graph goes out of scope. I will look into these separately.
Fixes https://github.com/pytorch/pytorch/issues/50454
Pull Request resolved: https://github.com/pytorch/pytorch/pull/51951
Reviewed By: mrshenli
Differential Revision: D26352629
Pulled By: SplitInfinity
fbshipit-source-id: 67299b6cbf1ac7ab77f8703a0ca8f1162e03fcd4
