fix PicoDet train bug (#4439)

configs/picodet/README.md

@@ -195,6 +195,15 @@ python tools/post_quant.py -c configs/picodet/picodet_s_320_coco.yml \
 
 </details>
 
+## FAQ
+
+<details>
+<summary>Out of memory error.</summary>
+
+Please reduce the `batch_size` of `TrainReader` in config.
+
+</details>
+
 ## Cite PP-PiocDet
 If you use PiocDet in your research, please cite our work by using the following BibTeX entry:
 ```

ppdet/modeling/architectures/picodet.py

@@ -75,7 +75,7 @@ class PicoDet(BaseArch):
     def get_loss(self, ):
         loss = {}
 
-        head_outs = self._forward()
+        head_outs, _ = self._forward()
         loss_gfl = self.head.get_loss(head_outs, self.inputs)
         loss.update(loss_gfl)
         total_loss = paddle.add_n(list(loss.values()))