Drop unused imports from caffe2/python (#49980)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/49980
From
```
./python/libcst/libcst codemod remove_unused_imports.RemoveUnusedImportsWithGlean --no-format caffe2/
```
Test Plan: Standard sandcastle tests
Reviewed By: xush6528
Differential Revision: D25727359
fbshipit-source-id: c4f60005b10546423dc093d31d46deb418352286
