Add a tutorial on proper benchmarking (#734)
* Performance tut
* toc
* Apply suggestions from code review - Tips will be the death of me
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
