Fix #3305: run_ner only possible on ModelForTokenClassification models
Also, #3305 breaks (if i'm not mistaken) the ability to run the example script from a pip-installed instance of transformers (vs. from an instance installed from source) (This PR does not fix that second issue)
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3335?src=pr&el=h1) Report
> Merging [#3335](https://codecov.io/gh/huggingface/transformers/pull/3335?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/20139b7c8d88f380f1e4e0ae2baf0b0ac9351039?src=pr&el=desc) will **decrease** coverage by `0.06%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/3335/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/3335?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #3335      +/-   ##
==========================================
- Coverage   77.63%   77.57%   -0.07%     
==========================================
  Files         100      100              
  Lines       16943    16943              
==========================================
- Hits        13154    13143      -11     
- Misses       3789     3800      +11
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/3335?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3335/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `86.4% <0%> (-1.97%)` | :arrow_down: |

------

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<|||||>Ok I'll merge this @srush @LysandreJik as it's more correct (feel free to let me know of your feedback anyways)<|||||>Thanks. Why does it break the pip version?<|||||>Because `MODEL_MAPPING` from `modeling_auto` is not exposed in the package's [`__init__.py`](https://github.com/huggingface/transformers/blob/master/src/transformers/__init__.py)