Fix missing id2label and label2id in run_ner.py
This is to retain the NER labels when training, so it can be used to map the labels during later prediction.

This functionality is present in the old version [https://github.com/huggingface/transformers/blob/master/examples/legacy/token-classification/run_ner.py#L170](https://github.com/huggingface/transformers/blob/master/examples/legacy/token-classification/run_ner.py#L170), but missing in the current one.

@sgugger 
This has just been done in #12001 :-)
Thanks for the PR!