configuration_utils: fix handling of `id2labels`
# What does this PR do?

The parameter `id2labels` of class `PretrainedConfig` is documented as `List[str]`, so enumerate() should be used rather
than dict.items() in the constructor.

Since a lot of code (including test code) passes `id2labels` as a dict, enumerate() is only used if it is a list indeed.
Hi, as you can see in [this example NER configuration file](https://s3.amazonaws.com/models.huggingface.co/bert/dslim/bert-base-NER/config.json), the `id2label` and `label2id` are actually dictionaries. 

Instead of doing the change you propose, changing the documentation to reflect that would be better. Thank you!<|||||>Wow, seems like someone else already fixed the documentation in the mean time.