Question Answering for TF trainer
This PR add the Question Answering task in the Tensorflow trainer. For the moment the evaluation is not possible due to the complexity of the SQuAD metrics but the training is working.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4320?src=pr&el=h1) Report
> Merging [#4320](https://codecov.io/gh/huggingface/transformers/pull/4320?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/4bf5042240d33286460b83f3dbf9be77500faab3&el=desc) will **decrease** coverage by `0.00%`.
> The diff coverage is `0.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4320/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4320?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4320      +/-   ##
==========================================
- Coverage   78.16%   78.15%   -0.01%     
==========================================
  Files         120      120              
  Lines       20005    20009       +4     
==========================================
+ Hits        15636    15638       +2     
- Misses       4369     4371       +2     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4320?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/trainer\_tf.py](https://codecov.io/gh/huggingface/transformers/pull/4320/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmVyX3RmLnB5) | `17.75% <0.00%> (-0.34%)` | :arrow_down: |
| [src/transformers/training\_args\_tf.py](https://codecov.io/gh/huggingface/transformers/pull/4320/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmluZ19hcmdzX3RmLnB5) | `55.31% <ø> (ø)` | |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4320/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `92.77% <0.00%> (ø)` | |
| [src/transformers/trainer.py](https://codecov.io/gh/huggingface/transformers/pull/4320/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmVyLnB5) | `41.09% <0.00%> (+0.28%)` | :arrow_up: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4320/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `73.85% <0.00%> (+0.41%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/4320?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4320?src=pr&el=footer). Last update [4bf5042...2a6162b](https://codecov.io/gh/huggingface/transformers/pull/4320?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>LMK when it's ready to merge<|||||>Good to merge!