Update gfl_head.py (#4210)
Update gfl_head.py (#4210)

* Update gfl_head.py

There is a error in pytorch1.6 version when call reduce_mean in this head: input tensor is "int" format, can not call div_ direct. so, change it to float format.

* Update gfl_head.py
open-mmlab/mmdetection,open-mmlab/mmdetection