Add docstring to DDPOptimizer

[wconstab]

Stack from ghstack (oldest at bottom):

• Enable DDPOptimizer by default in dynamo #88523
• -> Add docstring to DDPOptimizer #88521
• Add single-process DDP accuracy support to dynamo benchmark suite #88511
• DDPOptimizer replace debug=True/False with using torchdynamo logger #88480
• Support DDP ignored parameters in DDPOptimizer #88460
• Add hf_bert + DDP multigpu test #88435

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

[pytorch-bot]

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❌ 2 Failures

As of commit 94e1638:

The following jobs have failed:

• cuda11.6-py3.10-gcc7-sm86 / test (default, 2, 4, linux.g5.4xlarge.nvidia.gpu)
• cuda11.6-py3.10-gcc7-sm86 / test (inductor, 7, 7, linux.g5.4xlarge.nvidia.gpu)

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[wconstab]

@pytorchbot merge

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[pytorchmergebot]

Merge failed

Reason: 4 additional jobs have failed, first few of them are: inductor ,inductor / cuda11.6-py3.10-gcc7-sm86 / test (inductor, 7, 7, linux.g5.4xlarge.nvidia.gpu) ,trunk ,trunk / cuda11.6-py3.10-gcc7-sm86 / test (default, 2, 4, linux.g5.4xlarge.nvidia.gpu)

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[wconstab]

@pytorchbot merge -f "flaky CI"

[pytorchmergebot]

Merge started

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