text: Title: keyerror: all_points_y
  Body: I'm a beginner of deep-learning, recently, i try to use the mask-rcnn to make a project about rust detection. Firstly i watch the video how to use this code, then i modify the code of 'balloon', and have a try to train my own data, but i meet the error called keyerror: all_points_y, my dataset are many binary images which contain the rust areas(white) and the background(black). To approprate the requirements of code, i use the edge detection to found the outline of rust areas, then i save the class name(only 'rust' class) and the coordinate of the points which on the outlines as .json file according to the requirement format(firstly i save these data in a dict, then i use json.dump to transform the dict to json and save it):
      #{ 'filename': '28503151_5b5b7ec140_b.jpg',
          #   'regions': {
          #       '0': {
          #           'region_attributes': {},
          #           'shape_attributes': {
          #               'all_points_x': [...],
          #               'all_points_y': [...],
          #               'name': 'polygon'}},
          #       ... more regions ...
          #   },
          #   'size': 100202
          # }
  During the training processing, i saw some error messages repeatly about keyerror: all_point_y, the message repeat about 20 times when my train dataset has 120 pictures and the val dataset has 30 pictures. Easily see that not all picture will bring error, but i don't know how the error happened.
  I try to search the problem and only find this kind of answer: the points in dataset should be in an polygon instead of circle or rectangle, but the obviously the answer isn't fit for my question.
  Futher more, when load the json file, the order of data usually have change,for example, when i save the data, the format is:
   'shape_attributes': {
       'all_points_x': [...],
       'all_points_y': [...],
       'name': 'rust'}
  but when i load the data, it change to the format as:
       'all_points_y': [...],
       'name': 'rust',
       'all_points_y': [...]}
   whether this issue have an affect to the training processing?
  
  ps: 
  1.In fact , i'm also a beginner of english.
  2.Help me, every scholars(dalao), thanks·