torchvision QAT tutorial: update for QAT with DDP (#2280)
torchvision QAT tutorial: update for QAT with DDP (#2280)

Summary:

We've made two recent changes to QAT in PyTorch core:
1. add support for SyncBatchNorm
2. make eager mode QAT prepare scripts respect device affinity

This PR updates the torchvision QAT reference script to take
advantage of both of these.  This should be landed after
https://github.com/pytorch/pytorch/pull/39337 (the last PT
fix) to avoid compatibility issues.

Test Plan:

```
python -m torch.distributed.launch
  --nproc_per_node 8
  --use_env
  references/classification/train_quantization.py
  --data-path {imagenet1k_subset}
  --output-dir {tmp}
  --sync-bn
```

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Subscribers:

Tasks:

Tags:
pytorch/vision,pytorch/vision,pytorch/vision,pytorch/vision,pytorch/vision,pytorch/vision