[JIT] Add aten::slice optimization (#63049)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/63049
Given a graph produced from a function like this:
```
def foo():
li = [1, 2, 3, 4, 5, 6]
return li[0:2]
```
This pass produces a graph like this:
```
def foo():
li = [1, 2]
return li
```
These changes are mostly adapted from https://github.com/pytorch/pytorch/pull/62297/
Test Plan: `buck test //caffe2/test:jit -- TestPeephole`
Reviewed By: eellison
Differential Revision: D30231044
fbshipit-source-id: d12ee39f68289a574f533041a5adb38b2f000dd5
