ema update (#5089)

ppdet/optimizer.py

@@ -17,6 +17,7 @@ from __future__ import division
 from __future__ import print_function
 
 import math
+import weakref
 import paddle
 import paddle.nn as nn
 
@@ -319,19 +320,31 @@ class ModelEMA(object):
         self.use_thres_step = use_thres_step
         self.cycle_epoch = cycle_epoch
 
+        self._model_state = {
+            k: weakref.ref(p)
+            for k, p in model.state_dict().items()
+        }
+
     def reset(self):
         self.step = 0
         self.epoch = 0
         for k, v in self.state_dict.items():
             self.state_dict[k] = paddle.zeros_like(v)
 
-    def update(self, model):
+    def update(self, model=None):
         if self.use_thres_step:
             decay = min(self.decay, (1 + self.step) / (10 + self.step))
         else:
             decay = self.decay
         self._decay = decay
+
+        if model is not None:
             model_dict = model.state_dict()
+        else:
+            model_dict = {k: p() for k, p in self._model_state.items()}
+            assert all(
+                [v is not None for _, v in model_dict.items()]), 'python gc.'
+
         for k, v in self.state_dict.items():
             v = decay * v + (1 - decay) * model_dict[k]
             v.stop_gradient = True