Assert --image-weights not combined with DDP (ultralytics#3275)

train.py

@@ -525,6 +525,7 @@ def train(hyp, opt, device, tb_writer=None):
         device = torch.device('cuda', opt.local_rank)
         dist.init_process_group(backend='nccl', init_method='env://')  # distributed backend
         assert opt.batch_size % opt.world_size == 0, '--batch-size must be multiple of CUDA device count'
+        assert not opt.image_weights, '--image-weights argument is not compatible with DDP training'
         opt.batch_size = opt.total_batch_size // opt.world_size
 
     # Hyperparameters