Stack from ghstack (oldest at bottom):

• Update DDP docs for Dynamo/DDPOptimizer #89096
• -> Enable DDPOptimizer by default in dynamo #88523

Performance benchmarks on 6 popular models from 1-64 GPUs compiled with
torchinductor show performance gains or parity with eager, and showed
regressions without DDPOptimizer. *Note: resnet50 with small batch size shows a regression with optimizer, in part due to failing to compile one subgraph due to input mutation, which will be fixed.
(hf_Bert, hf_T5_large, hf_T5, hf_GPT2_large, timm_vision_transformer, resnet50)

Correctness checks are implemented in CI (test_dynamo_distributed.py),
via single-gpu benchmark scripts iterating over many models
(benchmarks/dynamo/torchbench.py/timm_models.py/huggingface.py),
and via (multi-gpu benchmark scripts in torchbench)[https://github.com/pytorch/benchmark/tree/main/userbenchmark/ddp_experiments].

cc @mlazos @soumith @voznesenskym @yanboliang @penguinwu @anijain2305 @EikanWang @jgong5 @Guobing-Chen @chunyuan-w @XiaobingSuper @zhuhaozhe @blzheng @Xia-Weiwen @wenzhe-nrv @jiayisunx @desertfire