text: Title: Error on mxnet/gluon: after upgrade the mxnet-cu100 and gluoncv,then run script "train_faster_rcnn.py"
  Body: My virtual environment is broken and then I create a new virtual environment named  "huf_mx" as  following.
  
  ![image](https://user-images.githubusercontent.com/47581857/69799144-ac21ea00-120d-11ea-8ecb-5367dc9a7fce.png)
  
  Trying to run script "train_faster_rcnn.py" on spyder IDE, I get the error.
  As shown in the image, there are some problem on mxnet/gluon, or maybe gluoncv either.
  
  How to fix it, installing another mxnet?
  I have tried 1.4.0,but not work.
  
  thx a lot.