[Fix] Input previous results for the last cascade_decode_head (open-mmlab#1450)

* [Fix] Input previous results for the latter cascade_decode_head

* minors

Author: MeowZheng

mmseg/models/segmentors/cascade_encoder_decoder.py

@@ -75,8 +75,12 @@ def _decode_head_forward_train(self, x, img_metas, gt_semantic_seg):
 
         for i in range(1, self.num_stages):
             # forward test again, maybe unnecessary for most methods.
-            prev_outputs = self.decode_head[i - 1].forward_test(
-                x, img_metas, self.test_cfg)
+            if i == 1:
+                prev_outputs = self.decode_head[0].forward_test(
+                    x, img_metas, self.test_cfg)
+            else:
+                prev_outputs = self.decode_head[i - 1].forward_test(
+                    x, prev_outputs, img_metas, self.test_cfg)
             loss_decode = self.decode_head[i].forward_train(
                 x, prev_outputs, img_metas, gt_semantic_seg, self.train_cfg)
             losses.update(add_prefix(loss_decode, f'decode_{i}'))