'Constructor of Microsoft COCO helper class for reading and visualizing annotations.
:param annotation_file (str): location of annotation file
:param image_folder (str): location to the folder that hosts images.
:return:'

def __init__(self, annotation_file=None):

    (self.dataset, self.anns, self.cats, self.imgs) = (dict(), dict(), dict(), dict())
    (self.imgToAnns, self.catToImgs) = (defaultdict(list), defaultdict(list))
    if (not (annotation_file == None)):
        print 'loading    annotations    into    memory...'
        tic = time.time()
        dataset = json.load(open(annotation_file, 'r'))
        assert (type(dataset) == dict), 'annotation    file    format    {}    not    supported'.format(type(dataset))
        print 'Done    (t={:0.2f}s)'.format((time.time() - tic))
        self.dataset = dataset
        self.createIndex()
