only sync cuda ifn cuda available (#3674)

Co-authored-by: Vasilis Vryniotis <datumbox@users.noreply.github.com>

references/detection/engine.py

@@ -84,6 +84,7 @@ def evaluate(model, data_loader, device):
     for images, targets in metric_logger.log_every(data_loader, 100, header):
         images = list(img.to(device) for img in images)
 
+        if torch.cuda.is_available():
             torch.cuda.synchronize()
         model_time = time.time()
         outputs = model(images)