Fix ddp+ema+qat flow (#110)

train.py

@@ -215,7 +215,7 @@ def train(hyp,  # path/to/hyp.yaml or hyp dictionary
             LOGGER.info('%s has been trained for %g epochs. Fine-tuning for %g additional epochs.' %
                         (weights, start_epoch-1, epochs))
             epochs += start_epoch  # finetune additional epochs
-        if sparseml_wrapper.qat_active(start_epoch):
+        if sparseml_wrapper.qat_active(start_epoch) and ema:
             ema.enabled = False
 
         # Optimizer
@@ -353,7 +353,8 @@ def train(hyp,  # path/to/hyp.yaml or hyp dictionary
             LOGGER.info('Disabling half precision and EMA, QAT scheduled to run')
             half_precision = False
             scaler._enabled = False
-            ema.enabled = False
+            if ema:
+                ema.enabled = False
         model.train()
 
         # Update image weights (optional, single-GPU only)