title: Adding images with no bounding box to tflite model maker object detection dataset
body: <p>I want to add images without any bounding boxes to the dataset used to train an object detector using tflite model maker.</p>
  <p>According to the <a href="https://cloud.google.com/vision/automl/object-detection/docs/csv-format" rel="nofollow noreferrer">docs</a> I can add</p>
  <blockquote>
  <p>one row for each image with no bounding box (such as row 4 below).</p>
  </blockquote>
  <pre><code> TRAIN,gs://folder/image1.png,car,0.1,0.1,,,0.3,0.3,,
   TRAIN,gs://folder/image1.png,bike,.7,.6,,,.8,.9,,
   UNASSIGNED,gs://folder/im2.png,car,0.1,0.1,0.2,0.1,0.2,0.3,0.1,0.3
   TEST,gs://folder/im3.png,,,,,,,,,
  </code></pre>
  <p>When doing so I am getting a ValueError &quot;could not convert string to float&quot;, which is caused by an attempt to cast the None object to a float in the following <a href="https://github.com/tensorflow/examples/blob/c930d0af2983ee438dcc005f9373188734f1624f/tensorflow_examples/lite/model_maker/core/data_util/object_detector_dataloader_util.py#L337-L338" rel="nofollow noreferrer">line</a>:</p>
  <pre><code>    xmin, ymin = float(line[3]) * width, float(line[4]) * height
  </code></pre>
  <p>Does anyone know how to properly add such images without bounding boxes? Are these empty images even helpful towards training the model?</p>