Clean up some type annotations in benchmarks/fastrnns (#49946)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/49946
Upgrades type annotations from Python2 to Python3
Test Plan: Sandcastle tests
Reviewed By: xush6528
Differential Revision: D25717510
fbshipit-source-id: 4f6431d140e3032b4ca55587f9602aa0ea38c671
