"config.json" does not include correct "id2label" and "label2id" after finetuning on NER task
## 🐛 Bug

<!-- Important information -->

Model I am using (Bert, XLNet....): xlmroberta

Language I am using the model on (English, Chinese....): English

The problem arise when using:
* [ ] the official example scripts: (give details)
I use script `run_ner.py` in order to finetune xlmroberta on conll03 dataset.

The script executed with no problems. But the file "config.json" in the output directory is not correct.


The tasks I am working on is:
* [ ] an official GLUE/SQUaD task: (give the name)
Task: NER
Dataset:  conll03


## To Reproduce

Steps to reproduce the behavior:

1.
I run the script: run_ner.py as follows:
`python run_ner.py --data_dir 0-data --model_type 'xlmroberta' --model_name_or_path 'xlm-roberta-large' --output_dir 1-out --max_seq_length 32 --do_train --do_eval --per_gpu_train_batch_size 8 --no_cuda --evaluate_during_training --logging_steps 1756 --save_steps 1756 --eval_all_checkpoints`
2. Go to the output directory. The file "config.json"  contains :
"label2id": {
    "LABEL_0": 0,
    "LABEL_1": 1
  },
and
"id2label": {
    "0": "LABEL_0",
    "1": "LABEL_1"
  },
which are not expected in NER
3.

<!-- If you have a code sample, error messages, stack traces, please provide it here as well. -->

## Expected behavior

<!-- A clear and concise description of what you expected to happen. -->
I expect that "config.json" contains something like:
"id2label": {
    "0": "B-LOC",
    "1": "B-MISC",
    "2": "B-ORG",
    "3": "I-LOC",
    "4": "I-MISC",
    "5": "I-ORG",
    "6": "I-PER",
    "7": "O"
  },
and
"label2id": {
    "B-LOC": 0,
    "B-MISC": 1,
    "B-ORG": 2,
    "I-LOC": 3,
    "I-MISC": 4,
    "I-ORG": 5,
    "I-PER": 6,
    "O": 7
  },
## Environment

* OS: Ubuntu 18.04
* Python version: 3.6.9
* PyTorch version: 1.3.1
* PyTorch Transformers version (or branch): 2.3.0
* Using GPU ? No
* Distributed or parallel setup ? No
* Any other relevant information:

## Additional context

<!-- Add any other context about the problem here. -->

I checked the codes days before and 'label2id' and 'id2label' seemed not used and didn't influence the code execution.