In the run_ner.py example, give the optional label arg a default value
Otherwise, if label is not specified, the following error occurs:

	Traceback (most recent call last):
	  File "run_ner.py", line 303, in <module>
	    main()
	  File "run_ner.py", line 101, in main
	    model_args, data_args, training_args = parser.parse_json_file(json_file=os.path.abspath(sys.argv[1]))
	  File "/home/user/anaconda3/envs/bert/lib/python3.7/site-packages/transformers/hf_argparser.py", line 159, in parse_json_file
	    obj = dtype(**inputs)
	TypeError: __init__() missing 1 required positional argument: 'labels'
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5326?src=pr&el=h1) Report
> Merging [#5326](https://codecov.io/gh/huggingface/transformers/pull/5326?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/5543b30aa6b52da3c8f7d9e525b0edc26226d717&el=desc) will **increase** coverage by `0.28%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5326/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5326?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5326      +/-   ##
==========================================
+ Coverage   77.42%   77.70%   +0.28%     
==========================================
  Files         138      138              
  Lines       24284    24284              
==========================================
+ Hits        18803    18871      +68     
+ Misses       5481     5413      -68     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5326?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_auto.py](https://codecov.io/gh/huggingface/transformers/pull/5326/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fYXV0by5weQ==) | `95.55% <0.00%> (-2.23%)` | :arrow_down: |
| [src/transformers/configuration\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5326/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX3V0aWxzLnB5) | `95.65% <0.00%> (-0.73%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5326/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `87.48% <0.00%> (+1.32%)` | :arrow_up: |
| [src/transformers/tokenization\_albert.py](https://codecov.io/gh/huggingface/transformers/pull/5326/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fYWxiZXJ0LnB5) | `87.50% <0.00%> (+58.65%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/5326?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5326?src=pr&el=footer). Last update [5543b30...bb7d305](https://codecov.io/gh/huggingface/transformers/pull/5326?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>LGTM!