Change graph_def file opening mode from to 'rb' (#6071)

As discussed here: https://github.com/tensorflow/tensorflow/issues/11312, this change allows this script to work in Python 3.

research/object_detection/inference/detection_inference.py → research/object_detection/inference/deetection_inference.py

@@ -65,7 +65,7 @@ def build_inference_graph(image_tensor, inference_graph_path):
     detected_labels_tensor: Detected labels. Int64 tensor,
         shape=[num_detections]
   """
-  with tf.gfile.Open(inference_graph_path, 'r') as graph_def_file:
+  with tf.gfile.Open(inference_graph_path, 'rb') as graph_def_file:
     graph_content = graph_def_file.read()
   graph_def = tf.GraphDef()
   graph_def.MergeFromString(graph_content)