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>