Fix loading of question answering bert from tf weights.
I've got an attribute error when loading pretrained tf weights for question answering (bert) in `load_tf_weights_in_bert` at:
```
elif l[0] == 'squad':
     pointer = getattr(pointer, 'classifier')
```
since `BertForQuestionAnswering` does not have a 'classifier' attribute but `qa_outputs`. I've added a try except, which resolves the error.  
# [Codecov](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1197?src=pr&el=h1) Report
> Merging [#1197](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1197?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/pytorch-transformers/commit/89fd3450a61b5efd76d2524df2454e0a0e4ca070?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1197/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1197?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #1197   +/-   ##
=======================================
  Coverage   80.83%   80.83%           
=======================================
  Files          46       46           
  Lines        7878     7878           
=======================================
  Hits         6368     6368           
  Misses       1510     1510
```


| [Impacted Files](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1197?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pytorch\_transformers/modeling\_bert.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1197/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvbW9kZWxpbmdfYmVydC5weQ==) | `88.03% <100%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1197?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/pytorch-transformers/pull/1197?src=pr&el=footer). Last update [89fd345...d6fb182](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1197?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.
