add stage_loss_weights (#6350)

* add stage_loss_weights

ppdet/modeling/heads/cascade_head.py

@@ -161,7 +161,9 @@ class CascadeHead(BBoxHead):
                               [30.0, 30.0, 15.0, 15.0]],
                  num_cascade_stages=3,
                  bbox_loss=None,
-                 reg_class_agnostic=True):
+                 reg_class_agnostic=True,
+                 stage_loss_weights=None):
+
         nn.Layer.__init__(self, )
         self.head = head
         self.roi_extractor = roi_extractor
@@ -173,6 +175,12 @@ class CascadeHead(BBoxHead):
         self.bbox_weight = bbox_weight
         self.num_cascade_stages = num_cascade_stages
         self.bbox_loss = bbox_loss
+        self.stage_loss_weights = [
+            1. / num_cascade_stages for _ in range(num_cascade_stages)
+        ] if stage_loss_weights is None else stage_loss_weights
+        assert len(
+            self.stage_loss_weights
+        ) == num_cascade_stages, f'stage_loss_weights({len(self.stage_loss_weights)}) do not equal to num_cascade_stages({num_cascade_stages})'
 
         self.reg_class_agnostic = reg_class_agnostic
         num_bbox_delta = 4 if reg_class_agnostic else 4 * num_classes
@@ -249,7 +257,7 @@ class CascadeHead(BBoxHead):
                                            self.bbox_weight[stage])
                 for k, v in loss_stage.items():
                     loss[k + "_stage{}".format(
-                        stage)] = v / self.num_cascade_stages
+                        stage)] = v * self.stage_loss_weights[stage]
 
             return loss, bbox_feat
         else: