Fix "save binary size" steps (#43529)
Summary:
`pip3` alias might not be available, so call `python3 -mpip` to be on the safe side
Should fix failures like that:
https://app.circleci.com/pipelines/github/pytorch/pytorch/203448/workflows/3837b2d6-b089-4a19-b797-38bdf989c82e/jobs/6913032/parallel-runs/0/steps/0-109
Pull Request resolved: https://github.com/pytorch/pytorch/pull/43529
Reviewed By: seemethere
Differential Revision: D23307306
Pulled By: malfet
fbshipit-source-id: b55e6782b29f1a1f56787902cbb85b3c3d20370c
