title: 'max-overlaps' issue in py-faster-rcnn
body: <p>I am trying to run <a href="https://github.com/rbgirshick/py-faster-rcnn" rel="nofollow noreferrer">py-faster-rcnn</a> on my own dataset following the tutorial.
  But it is giving me <strong>'Max_overlaps'</strong> issue. While over going through other issues reported in that repository, i found out out that it may arise if foreground or background roi's are missing.
  The format of Annotation file is exactly the same as mentioned here: 
  1 200 200 360 345
  while '1' being class name. and other values are x1,y1,x2 and y2 respectively.<br>
  Can anyone please point out that what can possibly cause this error?</p>
  
  <pre><code>File "./tools/train_net.py", line 112, in 
  max_iters=args.max_iters)
  File "/usr/local/caffes/xlw/faster-rcnn-third/tools/../lib/fast_rcnn/train.py", line 155, in train_net
  roidb = filter_roidb(roidb)
  File "/usr/local/caffes/xlw/faster-rcnn-third/tools/../lib/fast_rcnn/train.py", line 145, in filter_roidb
  filtered_roidb = [entry for entry in roidb if is_valid(entry)]
  File "/usr/local/caffes/xlw/faster-rcnn-third/tools/../lib/fast_rcnn/train.py", line 134, in is_valid
  overlaps = entry['max_overlaps']
  KeyError: 'max_overlaps'
  </code></pre>