[JIT] Add aten::slice optimization (#63049)

[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