add constant prop for immutable types (#30544)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/30544
Run Constant Propagation upon compilation only on ops with non-aliasing inputs and outputs. This speeds up the first run of `torchvision.models.resnet18` by over 50% and speeds up compilation by about 25% (although the effects didn't seem additive with with https://github.com/pytorch/pytorch/pull/30503, so I'm going to land this PR first and then see if caching still has a sizable impact).
Running constant prop only with non-aliasing types does a lot of graph cleanup by removing constant ifs and a bunch of other smaller ops. It also avoids all the jitter problems we had when we tried running full constant prop previously. Bc it is idempotent it doesn't jitter, and it doesn't jitter graphs constructed from tracing because tracing doesn't emit any ops that only involve non-aliasing inputs.
Full constant prop isn't idempotent because what ops are run depends on the state of mutation in alias db, which will often change upon successive iterations of constant propagation, and bc it affects graphs constructed from tracing.
Edit: if we were okay with running constant propagation on graphs constructed from tracing (potentially making them hard to debug), an alternative would be to run constant propagation until the graph reaches a fixed point.
Test Plan: Imported from OSS
Differential Revision: D18833607
Pulled By: eellison
fbshipit-source-id: 92a0adb4882d67ed5a0db5c279f5e122aeeba54a
