update transformers to 4.20.1 (#1025)
Summary:
- update transformers to 4.20.1
- remove patches
Pull Request resolved: https://github.com/pytorch/benchmark/pull/1025
Reviewed By: xuzhao9
Differential Revision: D37792741
Pulled By: FindHao
fbshipit-source-id: f3f378d2390742308b016e13f3e30f316ace222b
