title: normalization image before using ssd_mobilenet
body: <p>I try train ssd-mobilenet in my own dataset : 
  training image : 3400 with size :1600*1200 
  test set :800 with size :1600 *1200 tensorflow -gpu :1.13.1 gpu :4GB cuda 10.0 cudnn 7 
  object: road damage like aligator crack but after 197000 step my training loss cannot go down 2.
  I have 2 questions
  Should I normalize my training and set image before before using pretrained model like ssd_mobilenet?
  If yes
  Should I annotate images normalized or not ?
  I need really helps. Thanks in advance</p>