Fix bug in dataset reader. (#6871)

The ".mat" files loaded in the dataset are byte files. Python 3.7 requires them to be loaded using "rb".

research/delf/delf/python/detect_to_retrieve/dataset.py

@@ -40,7 +40,7 @@ def ReadDatasetFile(dataset_file_path):
       array of integers; additionally, it has a key 'bbx' mapping to a NumPy
       array of floats with bounding box coordinates.
   """
-  with tf.gfile.GFile(dataset_file_path, 'r') as f:
+  with tf.gfile.GFile(dataset_file_path, 'rb') as f:
     cfg = matlab.loadmat(f)
 
   # Parse outputs according to the specificities of the dataset file.