[Enhance] Raise warning for abnormal momentum (#655)

mmengine/model/averaged_model.py

 # Copyright (c) OpenMMLab. All rights reserved.
+import warnings
 from abc import abstractmethod
 from copy import deepcopy
 from typing import Optional
@@ -151,6 +152,13 @@ class ExponentialMovingAverage(BaseAveragedModel):
 
             Xema_{t+1} = (1 - momentum) * Xema_{t} +  momentum * X_t
 
+    .. note::
+        This :attr:`momentum` argument is different from one used in optimizer
+        classes and the conventional notion of momentum. Mathematically,
+        :math:`Xema_{t+1}` is the moving average and :math:`X_t` is the
+        new observed value. The value of momentum is usually a small number,
+        allowing observed values to slowly update the ema parameters.
+
     Args:
         model (nn.Module): The model to be averaged.
         momentum (float): The momentum used for updating ema parameter.
@@ -175,6 +183,12 @@ class ExponentialMovingAverage(BaseAveragedModel):
         super().__init__(model, interval, device, update_buffers)
         assert 0.0 < momentum < 1.0, 'momentum must be in range (0.0, 1.0)'\
                                      f'but got {momentum}'
+        if momentum > 0.5:
+            warnings.warn(
+                'The value of momentum in EMA is usually a small number,'
+                'which is different from the conventional notion of '
+                f'momentum but got {momentum}. Please make sure the '
+                f'value is correct.')
         self.momentum = momentum
 
     def avg_func(self, averaged_param: Tensor, source_param: Tensor,

tests/test_model/test_averaged_model.py

@@ -93,6 +93,13 @@ class TestAveragedModel(TestCase):
             model = torch.nn.Sequential(
                 torch.nn.Conv2d(1, 5, kernel_size=3), torch.nn.Linear(5, 10))
             ExponentialMovingAverage(model, momentum=3)
+
+        with self.assertWarnsRegex(
+                Warning,
+                'The value of momentum in EMA is usually a small number'):
+            model = torch.nn.Sequential(
+                torch.nn.Conv2d(1, 5, kernel_size=3), torch.nn.Linear(5, 10))
+            ExponentialMovingAverage(model, momentum=0.9)
         # test EMA
         model = torch.nn.Sequential(
             torch.nn.Conv2d(1, 5, kernel_size=3), torch.nn.Linear(5, 10))