Drop unused imports (#49972)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/49972
From
```
./python/libcst/libcst codemod remove_unused_imports.RemoveUnusedImportsWithGlean --no-format caffe2/
```
Test Plan: Standard sandcastle tests
Reviewed By: xush6528
Differential Revision: D25727352
fbshipit-source-id: 6b90717e161aeb1da8df30e67d586101d35d7d5f
