add add_special_tokens=True for input examples
Author: yaolu
Pull request: yes

According to #1957 , in some versions of transformers, add_special_tokens is not set to True by default. In that case, the example code is wrong as input_ids will missing [CLS] and [SEP] tokens. It's better to pass add_special_tokens=True in the example explicitly.

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/1971?src=pr&el=h1) Report
> Merging [#1971](https://codecov.io/gh/huggingface/transformers/pull/1971?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/5afca00b4732f57329824e1538897e791e02e894?src=pr&el=desc) will **increase** coverage by `0.18%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/1971/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/1971?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #1971      +/-   ##
==========================================
+ Coverage   84.06%   84.24%   +0.18%     
==========================================
  Files         105      104       -1     
  Lines       15536    15431     -105     
==========================================
- Hits        13060    13000      -60     
+ Misses       2476     2431      -45
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/1971?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [transformers/modeling\_bert.py](https://codecov.io/gh/huggingface/transformers/pull/1971/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX2JlcnQucHk=) | `87.77% <ø> (ø)` | :arrow_up: |
| [transformers/tests/modeling\_tf\_common\_test.py](https://codecov.io/gh/huggingface/transformers/pull/1971/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL3Rlc3RzL21vZGVsaW5nX3RmX2NvbW1vbl90ZXN0LnB5) | `95.63% <0%> (-1.46%)` | :arrow_down: |
| [transformers/configuration\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/1971/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL2NvbmZpZ3VyYXRpb25fdXRpbHMucHk=) | `92.1% <0%> (-0.11%)` | :arrow_down: |
| [transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/1971/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX3RmX3V0aWxzLnB5) | `92.68% <0%> (-0.05%)` | :arrow_down: |
| [transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/1971/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX3V0aWxzLnB5) | `89.43% <0%> (-0.03%)` | :arrow_down: |
| [transformers/configuration\_distilbert.py](https://codecov.io/gh/huggingface/transformers/pull/1971/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL2NvbmZpZ3VyYXRpb25fZGlzdGlsYmVydC5weQ==) | `89.74% <0%> (ø)` | :arrow_up: |
| [transformers/configuration\_auto.py](https://codecov.io/gh/huggingface/transformers/pull/1971/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL2NvbmZpZ3VyYXRpb25fYXV0by5weQ==) | `59.45% <0%> (ø)` | :arrow_up: |
| [transformers/tests/tokenization\_tests\_commons.py](https://codecov.io/gh/huggingface/transformers/pull/1971/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL3Rlc3RzL3Rva2VuaXphdGlvbl90ZXN0c19jb21tb25zLnB5) | `100% <0%> (ø)` | :arrow_up: |
| [transformers/modeling\_distilbert.py](https://codecov.io/gh/huggingface/transformers/pull/1971/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX2Rpc3RpbGJlcnQucHk=) | `95.87% <0%> (ø)` | :arrow_up: |
| [transformers/modeling\_auto.py](https://codecov.io/gh/huggingface/transformers/pull/1971/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX2F1dG8ucHk=) | `31.81% <0%> (ø)` | :arrow_up: |
| ... and [9 more](https://codecov.io/gh/huggingface/transformers/pull/1971/diff?src=pr&el=tree-more) | |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/1971?src=pr&el=footer). Last update [5afca00...d5dd44e](https://codecov.io/gh/huggingface/transformers/pull/1971?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).


This is less ambiguous, indeed! Thank you for taking the time to open a PR.