[docs] outline sharded ddp doc
This PR provides an initial outline of HF Trainer integration starting with ZeRO. We have fairscale's Sharded optimizer/gradients supported already and deepspeed is coming 

We won't merge this until fairscale merged all the required fixes and released a new version, but I thought it'd be good to start the doc going so it's ready when fairscale is ready.

I hope to submit a deepspeed integration shortly as well, so we will extend it then with deepspeed info. edit (https://github.com/huggingface/transformers/pull/9211)

@sgugger 