Add back option --do_lower_case in SQuAD for uncased models
The option `--do_lower_case` is currently needed by the uncased models (i.e., bert-base-uncased, bert-large-uncased). The results w/ and w/o this option are shown below. 

The option was deleted in the current v2.9.0 version so this PR tries to add it back. This would not affect the cased models (i.e., RoBERTa, XLNet).

Results of SQuAD v1.1:
bert-base-uncased **without** `--do_lower_case`:  'exact': 73.83, 'f1': 82.22
bert-base-uncased **with** `--do_lower_cas`e:  'exact': 81.02, 'f1': 88.34
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4245?src=pr&el=h1) Report
> Merging [#4245](https://codecov.io/gh/huggingface/transformers/pull/4245?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/7b75aa9fa55bee577e2c7403301ed31103125a35&el=desc) will **increase** coverage by `0.42%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4245/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4245?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4245      +/-   ##
==========================================
+ Coverage   78.39%   78.82%   +0.42%     
==========================================
  Files         120      120              
  Lines       19925    19925              
==========================================
+ Hits        15620    15705      +85     
+ Misses       4305     4220      -85     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4245?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4245/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `92.77% <0.00%> (-0.17%)` | :arrow_down: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4245/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `90.96% <0.00%> (-0.13%)` | :arrow_down: |
| [src/transformers/tokenization\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4245/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHMucHk=) | `89.48% <0.00%> (+0.11%)` | :arrow_up: |
| [src/transformers/data/processors/squad.py](https://codecov.io/gh/huggingface/transformers/pull/4245/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL3Byb2Nlc3NvcnMvc3F1YWQucHk=) | `56.27% <0.00%> (+27.65%)` | :arrow_up: |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4245?src=pr&el=footer). Last update [7b75aa9...f1caefd](https://codecov.io/gh/huggingface/transformers/pull/4245?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
