[JIT] Normalize op aliases (#38735)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/38735
Follow up to my comment https://github.com/pytorch/pytorch/pull/36597/#issuecomment-613674329
This adds a pass to convert op aliases into a normalized form. Having two ops generated in our IR that do the same thing makes the IR harder for downstream consumers of the IR, such as TorchScript passes but also ONNX, glow, etc.
Another solution would have been to fix our code generation to only emit `aten::abs` from the start. This seems trickier, and doesn't really buy us much if we still have to expose `aten::absolute` in C++, as glaringlee of the C++ API thinks we should.
Bike shedding: maybe this should be `CanonicalizeOps` instead
Test Plan: Imported from OSS
Differential Revision: D21673108
Pulled By: eellison
fbshipit-source-id: c328618907de1af22e07f57fd27fa619978c2817
