Configurations should support id2label as a list

[sijunhe]

Feature request

This is a rather minor/trivial feature request.

Currently id2label is of type Dict[int, str] in PretrainedConfig. Since this map is used to map class ids to labels, the keys is set to range(0, num_labels). Essentially, a list would also work here. I can even argue that a list is better in this case, because it avoids the potential error of the out-of-range keys and the weird serialization as

"id2label": {
    "0": "O",
    "1": "B-address",
    "2": "I-address",
    ...
}

Given that this attribute is mostly used for lookup id2label[i], it should be pretty easy to support this field both as a list and a map. We just need to fix a few places where we assumed .items() API exists.

I'll be happy to contribute a small PR for this.

Motivation

described above

Your contribution

Happy to contribute a PR for this.

[LysandreJik]

I would personally advocate for a single way of doing things, rather than many different ways of doing the same thing. In this situation, while the dict is more complex, it is also more complete.

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