message: fix InvalidArgumentError when using cv2 with tf.py_func()
diff: @@ -537,7 +537,7 @@ import cv2
  # Use a custom OpenCV function to read the image, instead of the standard
  # TensorFlow `tf.read_file()` operation.
  def _read_py_function(filename, label):
  - image_decoded = cv2.imread(image_string, cv2.IMREAD_GRAYSCALE)
  + image_decoded = cv2.imread(filename.decode(), cv2.IMREAD_GRAYSCALE)
  return image_decoded, label
  # Use standard TensorFlow operations to resize the image to a fixed shape.
  