Support for OIDv6

[zaichang]

They seem to have renamed some of the files instead of providing a separate base URL, for example oidv6-train-annotations-bbox.csv instead of train-annotations-bbox.csv

It would be nice to have an argument to choose OIDv6 (or perhaps set that as default)

[zaichang]

Also annoyingly, the validation and test csvs seem to remain unchanged and point to the v5 URL.

A temporary solution is to simply create a duplicate file with the expected name in csv_dir after downloading them cp oidv6-train-annotations-bbox.csv train-annotations-bbox.csv