[docs] outline sharded ddp doc (#9208)
* outline sharded dpp doc
* fix link
* add example
* Apply suggestions from code review
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
* narrow the command and remove non-essentials
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
