fix bug of mse_loss (#2301)
fix bug of mse_loss (#2301)

* fix bug of mse_loss

* reformat

* use single quote

Co-authored-by: penghaozhou <9a90f9d07864d2ce8b073952cbd7303407f40369@tencent.com>
Co-authored-by: wwzhang <326f1d416e55ccd8c3a0c323ace7b9b1eefcfc7a@outlook.com>
open-mmlab/mmdetection,open-mmlab/mmdetection