[docs] outline sharded ddp doc (#9208)
[docs] outline sharded ddp doc (#9208)

* outline sharded dpp doc

* fix link

* add example

* Apply suggestions from code review

Co-authored-by: Sylvain Gugger <222c4d44cc8ec39802c192ed0aa3f97cdb77ff4e@users.noreply.github.com>

* narrow the command and remove non-essentials

Co-authored-by: Sylvain Gugger <222c4d44cc8ec39802c192ed0aa3f97cdb77ff4e@users.noreply.github.com>
huggingface/transformers,huggingface/transformers,huggingface/transformers,huggingface/pytorch-transformers,huggingface/transformers