Fix bug for prefetcher

Set input and target to Cuda when without using prefetcher.

train.py

@@ -428,6 +428,9 @@ def validate(model, loader, loss_fn, args):
     with torch.no_grad():
         for batch_idx, (input, target) in enumerate(loader):
             last_batch = batch_idx == last_idx
+            if not args.prefetcher:
+                input = input.cuda()
+                target = target.cuda()
 
             output = model(input)
             if isinstance(output, (tuple, list)):