[fx] Add DCE pass (#52658)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/52658
DCE will reverse iterate over the graph looking for nodes without users and delete them. It will skip over unused placeholders (since this affects the signature of the method) and outputs (which never have users but we want to keep them :) )
Test Plan: Added unit tests
Reviewed By: jamesr66a, khabinov, chenccfb
Differential Revision: D26602212
fbshipit-source-id: f4f196973e40546076636090bb0008c24f33795e
