Remove apex.parallel. Use torch.nn.parallel

For future compatibility

train.py

@@ -7,6 +7,7 @@
 import torch.optim.lr_scheduler as lr_scheduler
 import torch.utils.data
 from torch.utils.tensorboard import SummaryWriter
+from torch.nn.parallel import DistributedDataParallel as DDP
 
 import test  # import test.py to get mAP after each epoch
 from models.yolo import Model
@@ -17,9 +18,7 @@
 mixed_precision = True
 try:  # Mixed precision training https://github.com/NVIDIA/apex
     from apex import amp
-    from apex.parallel import DistributedDataParallel as DDP
 except:
-    from torch.nn.parallel import DistributedDataParallel as DDP
     print('Apex recommended for faster mixed precision training: https://github.com/NVIDIA/apex')
     mixed_precision = False  # not installed
 
@@ -195,10 +194,7 @@ def train(hyp, tb_writer, opt, device):
     if device.type != 'cpu' and opt.local_rank != -1:
         # pip install torch==1.4.0+cku100 torchvision==0.5.0+cu100 -f https://download.pytorch.org/whl/torch_stable.html
         model = torch.nn.SyncBatchNorm.convert_sync_batchnorm(model)
-        if mixed_precision:
-            model = DDP(model, delay_allreduce=True)
-        else:
-            model = DDP(model, device_ids=[opt.local_rank])
+        model = DDP(model, device_ids=[opt.local_rank], output_device=opt.local_rank)
 
     # Model parameters
     hyp['cls'] *= nc / 80.  # scale coco-tuned hyp['cls'] to current dataset