[pytorch] Update caffe2/python to eliminate Pyre errors (#52083)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/52083
This makes minor fixes in `caffe2/python` to address all errors currently
reported by Pyre.
I update the code to fix errors when doing so looked simple and safe,
and added `pyre-fixme` comments in other places.
ghstack-source-id: 121109695
Test Plan: Confirmed that Pyre no longer reports errors under `caffe2/python`
Differential Revision: D26272279
fbshipit-source-id: b1eb19d323b613f23280ce9c71e800e874ca1162
