Update examples/ner/run_ner.py to use AutoModel
This PR is updating `run_ner.py` to use AutoModel implementation. Refer to #3290 and is simpler than before.

Maybe @srush can review this.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3305?src=pr&el=h1) Report
> Merging [#3305](https://codecov.io/gh/huggingface/transformers/pull/3305?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/b2028cc26b61a9dad960274d427e261af7c9bdc8&el=desc) will **decrease** coverage by `0.00%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/3305/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/3305?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #3305      +/-   ##
==========================================
- Coverage   77.47%   77.46%   -0.01%     
==========================================
  Files          99       99              
  Lines       16799    16799              
==========================================
- Hits        13015    13014       -1     
- Misses       3784     3785       +1     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/3305?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3305/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `85.68% <0.00%> (-0.36%)` | :arrow_down: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3305/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `93.70% <0.00%> (+0.13%)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3305?src=pr&el=footer). Last update [b2028cc...85e70d9](https://codecov.io/gh/huggingface/transformers/pull/3305?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
