Adding --do_lower_case option
Author: tkhs623
Pull request: yes

In SQuAD 1.1 using the uncased BERT model, **--do_lower_case** option must be used for initialization of a tokenizer.
Omitting this optional argument, the performance in development set would be lower around 8-9pt in both EM and F1 (In the official setting described in readme, the actual metrics were 72.09 (EM)/81.84 (F1) we confirmed).

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4000?src=pr&el=h1) Report
> Merging [#4000](https://codecov.io/gh/huggingface/transformers/pull/4000?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/4e817ff41885063e08bb3bcd63e5adfd835b9911&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/4000/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4000?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4000      +/-   ##
==========================================
- Coverage   78.44%   78.44%   -0.01%     
==========================================
  Files         111      111              
  Lines       18518    18518              
==========================================
- Hits        14527    14526       -1     
- Misses       3991     3992       +1     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4000?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4000/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `68.49% <0.00%> (-0.37%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4000/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `92.59% <0.00%> (-0.17%)` | :arrow_down: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4000/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `91.06% <0.00%> (+0.12%)` | :arrow_up: |

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There's no option to do `--do_lower_case` in the scripts anymore, the issue is that this particular tokenizer should have a `tokenizer_config.json` that always applies lowercasing.

The option `--do_lower_case` seems to be available in the [run_squad.py](https://github.com/huggingface/transformers/blob/master/examples/run_squad.py#L579-L581), currently (but not applied in the official readme).
It's my understanding that `--do_lower_case` option should be applied, in order to reproduce the official results described in readme (only uncased models' examples).

Or do you mean that `--do_lower_case` should not be used in the current (or future) version?

Yes, it shouldn’t be used in current or future versions of the scripts (because it’s handled in tokenizer configuration)

In your case, you should apply it manually for now, while we fix the root issue

Thank you for your answers!
I understand the issue and its solution, and so I will be closing this pull request.