Refactor subgraph merging (#44238)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/44238
Refactor create_autodiff_subgraphs to use the same updating of output aliasing properties logic as tensorexpr fuser, and factor that out to a common function in subgraph utils.
Test Plan: Imported from OSS
Reviewed By: Krovatkin, robieta
Differential Revision: D23871565
Pulled By: eellison
fbshipit-source-id: 72df253b16baf8e4aabf3d68b103b29e6a54d44c
