Fix scaler bug (#1323)

Author: natuan

src/sparseml/pytorch/torchvision/train.py

@@ -605,7 +605,8 @@ def log_metrics(tag: str, metrics: utils.MetricLogger, epoch: int, epoch_step: i
         if args.distributed:
             train_sampler.set_epoch(epoch)
         if manager is not None and manager.qat_active(epoch=epoch):
-            scaler._enabled = False
+            if scaler is not None:
+                scaler._enabled = False
             model_ema = None
 
         train_metrics = train_one_epoch(