text: Title: Faster R-CNN example bug report
  Body: On https://github.com/ppwwyyxx/tensorpack/blob/master/examples/FasterRCNN/train.py#L105 
  `decoded_boxes = decode_bbox_target(rpn_box_logits, fm_anchors) # fHxfWxNAx4, floatbox`
  the `decoded_boxes` is defined on the feature_map which is 1/16.0 scale smaller than original image, because `fm_anchors `is defined on feature_map but not the original image size.
  
  But in https://github.com/ppwwyyxx/tensorpack/blob/master/examples/FasterRCNN/train.py#L115
  The `rcnn_sampled_boxes `is also come from  `decoded_boxes  `and not resize, so it should define already on  1/16.0 scale smaller feature map, you should not multiply 1/16.0
    