title: What is loss_cls and loss_bbox and why are they always zero in training
body: <p>I'm trying to train a custom dataset on using <code>faster_rcnn</code> using the Pytorch implementation of Detectron <a href="https://github.com/roytseng-tw/Detectron.pytorch" rel="noreferrer">here</a>. I have made changes to the dataset and configuration according to the guidelines in the repo.</p>
  
  <p>The training process is carried out successfully, but the <code>loss_cls</code> and <code>loss_bbox</code> values are 0 from the beginning and even though the training is completed, final output cannot be used to make an evaluation or an inference.
  I would like to know what these two mean and how to get those values to change during the training. The exact model I'm using here is <code>e2e_faster_rcnn_R-50-FPN_1x</code></p>
  
  <p>Any help regarding this would be appreciated. I' using Ubuntu 16.04 with Python 3.6 on Anaconda, CUDA 9, cuDNN 7.</p>