Fix Pyre problems in executorch (#3955)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/3955

Current problems:

https://www.internalfb.com/phabricator/paste/view/P1410619602

We will eventually migrate from pyre to mypy at some point. But in the meantime, here's a fix.

Reviewed By: tarun292

Differential Revision: D58473224

fbshipit-source-id: a2e789b6c324c98d64c26f5ad5103de60dce513d