[doc] DP/PP/TP/etc parallelism (#12524)
* wip
* complete the doc
* missing img
* improve
* correction
* Apply suggestions from code review
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
