Make a random crop of (`size` x `size`).

def _random_crop(image, size):
  """Make a random crop of (`size` x `size`)."""
  bbox = tf.constant([0.0, 0.0, 1.0, 1.0], dtype=tf.float32, shape=[1, 1, 4])
  random_image, bbox = distorted_bounding_box_crop(
      image,
      bbox,
      min_object_covered=0.1,
      aspect_ratio_range=(3. / 4, 4. / 3.),
      area_range=(0.08, 1.0),
      max_attempts=1,
      scope=None)
  bad = _at_least_x_are_true(tf.shape(image), tf.shape(random_image), 3)

  image = tf.cond(
      bad, lambda: _center_crop(_do_scale(image, size), size),
      lambda: tf.image.resize_bicubic([random_image], [size, size])[0])
  return image