[JIT][SR] Introduce prim::IfThenElse (#72587)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/72587
This pattern frequently appears in a few graphs:
```
%result = prim::If(%condition)
block0():
-> (%a)
block1():
-> (%b)
```
This is slow, particularly in static runtime. Static runtime creates memory planners/block runners for each sub-block, which eats up a lot of memory and introduces a lot of extra overhead for this relatively simple operation.
This diff introduces a new op that replaces nodes like the above with a single op meant to act like a ternary operator:
```
%result = prim::IfThenElse(%condition, %a, %b)
```
Test Plan: New unit tests
Reviewed By: eellison
Differential Revision: D34091789
fbshipit-source-id: eb6a8c460c39b4c019a1f4ab1f3f1e5b6edc400c
(cherry picked from commit 0f1b335e5b83f402bda2dcdd9ecb411e0b67c651)
