fix tiny errors.

research/object_detection/utils/spatial_transform_ops.py

@@ -415,7 +415,7 @@ def multilevel_native_crop_and_resize(images, boxes, box_levels,
                                       crop_size, scope=None):
   #FIXME: fix docstring
   """doc string."""
-  if not box_levels:
+  if box_levels is None:
     return native_crop_and_resize(images[0], boxes, crop_size, scope=None)
   croped_feature_list = []
   for level, image in enumerate(images):
@@ -424,6 +424,7 @@ def multilevel_native_crop_and_resize(images, boxes, box_levels,
     croped_feature_list.append(cropped)
   return tf.concat(croped_feature_list, axis=0)
 
+
 def native_crop_and_resize(image, boxes, crop_size, scope=None):
   """Same as `matmul_crop_and_resize` but uses tf.image.crop_and_resize."""
   def get_box_inds(proposals):
@@ -443,6 +444,7 @@ def native_crop_and_resize(image, boxes, crop_size, scope=None):
                              tf.shape(cropped_regions)[1:]], axis=0)
     return tf.reshape(cropped_regions, final_shape)
 
+
 def multilevel_matmul_crop_and_resize(images, boxes, box_levels, crop_size, 
                                       extrapolation_value=0.0, scope=None):
   #FIXME: fix docstring
@@ -450,7 +452,7 @@ def multilevel_matmul_crop_and_resize(images, boxes, box_levels, crop_size,
   with tf.name_scope(scope, 'MatMulCropAndResize'):
     if box_levels is None:
       box_levels = tf.zeros(tf.shape(boxes)[:2], dtype=tf.int32)
-    return multilevel_roi_align([image],
+    return multilevel_roi_align(images,
                                 boxes,
                                 box_levels,
                                 crop_size,