layer name change to match compatibility with pytorch layer name in BertForQuestionAnswering
As pointed out in #438 When using BertForQuestionAnswering to load a tensorflow model using from_pretrained, one runs into an error 
`AttributeError: 'BertForQuestionAnswering' object has no attribute 'classifier'`

As pointed out in the thread it should be "qa_outputs" and not "classifier" for this functionality to work as expected. 

# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4458?src=pr&el=h1) Report
> Merging [#4458](https://codecov.io/gh/huggingface/transformers/pull/4458?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/384f0eb2f9d42e44094dbfd0917ccf4e6ddb462a&el=desc) will **decrease** coverage by `0.08%`.
> The diff coverage is `0.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4458/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4458?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4458      +/-   ##
==========================================
- Coverage   77.96%   77.88%   -0.09%     
==========================================
  Files         120      120              
  Lines       20140    20140              
==========================================
- Hits        15703    15686      -17     
- Misses       4437     4454      +17     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4458?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_bert.py](https://codecov.io/gh/huggingface/transformers/pull/4458/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19iZXJ0LnB5) | `88.82% <0.00%> (ø)` | |
| [src/transformers/data/processors/squad.py](https://codecov.io/gh/huggingface/transformers/pull/4458/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL3Byb2Nlc3NvcnMvc3F1YWQucHk=) | `28.66% <0.00%> (-5.42%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4458/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `88.50% <0.00%> (-0.17%)` | :arrow_down: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4458/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `73.85% <0.00%> (+0.41%)` | :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/4458?src=pr&el=footer). Last update [384f0eb...568d3f1](https://codecov.io/gh/huggingface/transformers/pull/4458?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
<|||||>I am not quite sure who the right person to review and include this PR would be. But commenting to open it up back nonetheless. 