callback_get_label no longer works as described

[zimonitrome]

callback_get_label: a callback-like function which takes two arguments - dataset and index

This no longer seems to be the case?

Please update how the new use-case looks like, because above commit breaks a lot of my previous code.

@hwany-j

[Harvey Wang]

same

[zhiqingS]

same

[baibizhe]

Same