Is building a training data set, from unlabeled data, for a machine learning classifier considered as a scientific contribution?
You will very likely not be able to publish such a labeling as a standalone journal article - it is simply too routine and does not advance scientific knowledge as such.
If this labeling is done as part of a larger machine learning project, the contribution would be the larger project.
If, finally, there was one person in a team whose main contribution to a larger ML project was labeling unlabeled data, this may be sufficient for coauthorship; especially if the labeling took a significant amount of time and if this person also actively reviewed the manuscript etc. This would need to be clarified with the other authors. It seems to be on a par with the question when someone is a coauthor whose main contributions were writing software, or doing the statistics.