GitLab

add model scaling section (#15119)

* add model scaling section

* Apply suggestions from code review
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

* integrate reviewer feedback

* initialize GPU properly

* add note about BnB optimizer

* move doc from `scaling.mdx` to `performance.mdx`

* integrate reviewer feedback

* revert section levels
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>