Support for basic list comprehensions (#17267)
Summary:
Supports the following syntax:
```
torch.jit.script
def comp(l):
# type: (List[float]) -> List[float]
n = [x * 3 for x in l]
return n
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/17267
Differential Revision: D14581119
Pulled By: Krovatkin
fbshipit-source-id: 6fd091a8a9ab607386ac58fda6ad88bf8aea380e
