I’m trying to applying apex.amp to  recent detection transformer (DETR) code (link 1)
What I’m not sure is where to put amp.initialize
Here are lines from the main.py of DETR where model and optimizer are declared (from line#121 1)
    model, criterion, postprocessors = build_model(args)
    model.to(device)

    model_without_ddp = model
    if args.distributed:
        model = torch.nn.parallel.DistributedDataParallel(model, device_ids=[args.gpu])
        model_without_ddp = model.module
    n_parameters = sum(p.numel() for p in model.parameters() if p.requires_grad)
    print('number of params:', n_parameters)

    param_dicts = [
        {"params": [p for n, p in model_without_ddp.named_parameters() if "backbone" not in n and p.requires_grad]},
        {
            "params": [p for n, p in model_without_ddp.named_parameters() if "backbone" in n and p.requires_grad],
            "lr": args.lr_backbone,
        },
    ]
    optimizer = torch.optim.AdamW(param_dicts, lr=args.lr,
                                  weight_decay=args.weight_decay)
    lr_scheduler = torch.optim.lr_scheduler.StepLR(optimizer, args.lr_drop)

According to the apex imagenet example, amp.initialize should place before wrapping model with DDP. I can move DDP wrapping part to after the lr_scheduler like below,
    model, criterion, postprocessors = build_model(args)
    model.to(device)

    model_without_ddp = model  

    param_dicts = [
        {"params": [p for n, p in model_without_ddp.named_parameters() if "backbone" not in n and p.requires_grad]},
        {
            "params": [p for n, p in model_without_ddp.named_parameters() if "backbone" in n and p.requires_grad],
            "lr": args.lr_backbone,
        },
    ]
    optimizer = torch.optim.AdamW(param_dicts, lr=args.lr,
                                  weight_decay=args.weight_decay)
    lr_scheduler = torch.optim.lr_scheduler.StepLR(optimizer, args.lr_drop)

    # Apex amp
    model, optimizer = amp.initialize(model, optimizer,
                                      opt_level=args.opt_level,
                                      keep_batchnorm_fp32=args.keep_batchnorm_fp32,
                                      loss_scale=args.loss_scale
                                      )
    
    if args.distributed:
        model = torch.nn.parallel.DistributedDataParallel(model, device_ids=[args.gpu])
        model_without_ddp = model.module
    n_parameters = sum(p.numel() for p in model.parameters() if p.requires_grad)
    print('number of params:', n_parameters)

But I wonder if I can pass model_without_ddp to amp.initialize after the end of above code without moving the DDP wrapping lines.
...
    optimizer = torch.optim.AdamW(param_dicts, lr=args.lr,
                                  weight_decay=args.weight_decay)
    lr_scheduler = torch.optim.lr_scheduler.StepLR(optimizer, args.lr_drop)
    
    # Apex amp
    model, optimizer = amp.initialize(model_without_ddp, optimizer,
                                      opt_level=args.opt_level,
                                      keep_batchnorm_fp32=args.keep_batchnorm_fp32,
                                      loss_scale=args.loss_scale
                                      )