Drop unused imports from leftovers (#49953)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/49953
From
```
./python/libcst/libcst codemod remove_unused_imports.RemoveUnusedImportsWithGlean --no-format caffe2/
```
Test Plan: Standard sandcastle tests
Reviewed By: xush6528
Differential Revision: D25727348
fbshipit-source-id: b3feef80b9b4b535f1bd4060dace5b1a50bd5e69
