json.decoder.JSONDecodeError: Extra data: line 1 column 3 (char 2)

错误原因是json文件太大，改为单行读取：

with open("COCO_train.json", "r+") as f:
    data = json.load(f)

改为：

with open("COCO_train.json", "r+") as f:
    data = f.readline()