'fill basic information to initialize imdb
:param image_set: train2014, val2014, test2015
:param root_path: \'data\', will write \'rpn_data\', \'cache\'
:param data_path: \'data/coco\''

def __init__(self, image_set, root_path, data_path):

    super(coco, self).__init__('COCO', image_set, root_path, data_path)
    self.root_path = root_path
    self.data_path = data_path
    self.coco = COCO(self._get_ann_file())
    cats = [cat['name'] for cat in self.coco.loadCats(self.coco.getCatIds())]
    self.classes = (['__background__'] + cats)
    self.num_classes = len(self.classes)
    self._class_to_ind = dict(zip(self.classes, xrange(self.num_classes)))
    self._class_to_coco_ind = dict(zip(cats, self.coco.getCatIds()))
    self._coco_ind_to_class_ind = dict([(self._class_to_coco_ind[cls], self._class_to_ind[cls]) for cls in self.classes[1:]])
    self.image_set_index = self._load_image_set_index()
    self.num_images = len(self.image_set_index)
    logger.info(('%s    num_images    %d' % (self.name, self.num_images)))
    view_map = {'minival2014': 'val2014', 'valminusminival2014': 'val2014'}
    self.data_name = (view_map[image_set] if (image_set in view_map) else image_set)
