Add support for simpler for-in-list + tests (#16726)
Summary:
This PR add supports for simpler for-in-list loops such as the example below:
```python
torch.ji.python
def sum_list(a):
# type: (List[int]) -> int
sum = 0
for i in a:
sum += i
return sum
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16726
Differential Revision: D14070007
Pulled By: ezyang
fbshipit-source-id: b4d971ee647729a6caa3099ceac34ec5c4f143de
