[PyTorch][Edge] Fix mypy error (#56999)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/56999
## Summary
Currently
## Test
![image](https://user-images.githubusercontent.com/16430979/116294682-19acaf80-a74d-11eb-9596-3a1d697ae835.png)
Note: there are still some other mypy failure for other functions in other repo
Differential Revision: D28023671
Test Plan:
See the test image above
Also CI
Reviewed By: dhruvbird
Pulled By: cccclai
fbshipit-source-id: d59da32b8b5a12c3f13bc5f4e02794db01132be3