Drop unused imports from scripts (#49956)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/49956
From
```
./python/libcst/libcst codemod remove_unused_imports.RemoveUnusedImportsWithGlean --no-format caffe2/
```
Test Plan: Standard sandcastle tests
Reviewed By: xush6528
Differential Revision: D25727347
fbshipit-source-id: 74d0a08aa0cfd0f492688a2b8278a0c65fd1deba
