[Enhance] Use torch.lerp_() to speed up EMA. (#519)

mmengine/model/averaged_model.py

@@ -187,8 +187,7 @@ class ExponentialMovingAverage(BaseAveragedModel):
             steps (int): The number of times the parameters have been
                 updated.
         """
-        averaged_param.mul_(1 - self.momentum).add_(
-            source_param, alpha=self.momentum)
+        averaged_param.lerp_(source_param, self.momentum)
 
 
 @MODELS.register_module()
@@ -242,4 +241,4 @@ class MomentumAnnealingEMA(ExponentialMovingAverage):
                 updated.
         """
         momentum = max(self.momentum, self.gamma / (self.gamma + self.steps))
-        averaged_param.mul_(1 - momentum).add_(source_param, alpha=momentum)
+        averaged_param.lerp_(source_param, momentum)