Open Images download hotfix

[brimoor]

Fixes a bug where label files were not being downloaded when running a load_zoo_dataset() call that does not include classes or attrs options in an environment where Open Images has never been downloaded:

import fiftyone.zoo as foz

foz.delete_zoo_dataset("open-images-v6", split="validation")

# previously would fail, now succeeds
dataset = foz.load_zoo_dataset("open-images-v6", split="validation", max_samples=5)

[benjaminpkane]

LGTM