docs wrt using accelerate launcher with trainer (#24250)
* update docs
* missing part
* Apply suggestions from code review
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
* address comments
* address Zach's comment
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Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
