text: Title: run demo_ssd.py gets error
  Body: ----------Python Info----------
  Version      : 3.5.6
  Compiler     : GCC 7.3.0
  Build        : ('default', 'Aug 26 2018 21:41:56')
  Arch         : ('64bit', '')
  ------------Pip Info-----------
  Version      : 10.0.1
  Directory    : /home/z440/miniconda3/envs/mxnet/lib/python3.5/site-packages/pip
  ----------MXNet Info-----------
  Version      : 1.6.0
  Directory    : /home/z440/miniconda3/envs/mxnet/lib/python3.5/site-packages/mxnet
  Commit Hash   : b1932c027ba8df081ca398dd8b5d3a893c5bc61d
  Library      : ['/home/z440/miniconda3/envs/mxnet/lib/python3.5/site-packages/mxnet/libmxnet.so']