Remove "-b" option from `pip install` command (#48742)
Summary:
It has been deprecated for a while and was finally removed in 20.3
Followup after https://github.com/pytorch/pytorch/pull/48722
Fixes ONNX build failures after docker image update
Pull Request resolved: https://github.com/pytorch/pytorch/pull/48742
Reviewed By: walterddr
Differential Revision: D25282017
Pulled By: malfet
fbshipit-source-id: 1dfa4eb57398f979107ca1544aafbc6d7b5e68a4
