Expand benchmark utils docs (#51664)
Summary:
Add some much needed documentation on the Timer callgrind output format, and expand what is shown on the website.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/51664
Reviewed By: tugsbayasgalan
Differential Revision: D26246675
Pulled By: robieta
fbshipit-source-id: 7a07ff35cae07bd2da111029242a5dc8de21403c
