Added "allow_new_labels" flag for class_labels tensor

🚀 🚀 Pull Request

Impact

[ ] Bug fix (non-breaking change which fixes expected existing functionality)
[X] Enhancement/New feature (adds functionality without impacting existing logic)
[ ] Breaking change (fix or feature that would cause existing functionality to change)

Description

By default, tensors with htype="class_label" will accept any new values added to them. This PR adds a new allow_new_labels=False setting in the tensor "info" which changes that behavior to instead throw an exception if an unknown label is added.

The available labels are set in the info["class_names"] setting, either when the tensor is originally created:

        ds.create_tensor(
            "labels",
            htype="class_label",
            class_names=["cat", "dog", "horse"],
            allow_new_labels=False,
        )

or set/updated later:

ds.labels.info.update(allow_new_labels=False)
ds.labels.info.update(class_names=["cat", "dog", "horse")

Things to be aware of

If you update the class_names to be a different order or skipping existing labels, the label_id->text mapping will be off and reading from the tensor will give you incorrect results.

activeloopai / deeplake