update flickr url option

[carat-jhcha]

hi
I use this library for our project
But there is some bug in there.
If I upload coco test dataset, it throw an error in read flickr_url.
Because, image_info_test2017.json(coco test json file) doesn't have flickr_url
could you merge this PR?

[cansik]

@jinhyuckcha I think it is easier to just use the parameter infer_missing. It would be great to have this flag in the load() method as well:

with open("dataset.json", "r") as f:
    dataset = ObjectDetectionDataset.from_json(f.read(), infer_missing=True)