Improved AutoBatch DDP error message (#6568)

* Improved AutoBatch DDP error message

* Cleanup

train.py

@@ -522,10 +522,12 @@ def main(opt, callbacks=Callbacks()):
     # DDP mode
     device = select_device(opt.device, batch_size=opt.batch_size)
     if LOCAL_RANK != -1:
+        msg = 'is not compatible with YOLOv5 Multi-GPU DDP training'
+        assert not opt.image_weights, f'--image-weights {msg}'
+        assert not opt.evolve, f'--evolve {msg}'
+        assert opt.batch_size != -1, f'AutoBatch with --batch-size -1 {msg}, please pass a valid --batch-size'
+        assert opt.batch_size % WORLD_SIZE == 0, f'--batch-size {opt.batch_size} must be multiple of WORLD_SIZE'
         assert torch.cuda.device_count() > LOCAL_RANK, 'insufficient CUDA devices for DDP command'
-        assert opt.batch_size % 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'
-        assert not opt.evolve, '--evolve argument is not compatible with DDP training'
         torch.cuda.set_device(LOCAL_RANK)
         device = torch.device('cuda', LOCAL_RANK)
         dist.init_process_group(backend="nccl" if dist.is_nccl_available() else "gloo")