Add special tokens to documentation for bert examples to resolve issue: #1561
Author: enzoampil
Pull request: yes

**Currently the BERT examples only show the strings encoded without the inclusion of special tokens (e.g. [CLS] and [SEP]) as illustrated below:**
```
tokenizer = BertTokenizer.from_pretrained('bert-base-uncased')
sentence = "Hello there, General Kenobi!"
print(tokenizer.encode(sentence))
print(tokenizer.cls_token_id, tokenizer.sep_token_id)

# [7592, 2045, 1010, 2236, 6358, 16429, 2072, 999]
# 101 102
```
**In this pull request i set add_special_tokens=True in order to include special tokens in the documented examples as illustrated below:**

```
tokenizer = BertTokenizer.from_pretrained('bert-base-uncased')
sentence = "Hello there, General Kenobi!"
print(tokenizer.encode(sentence, add_special_tokens=True))
print(tokenizer.cls_token_id, tokenizer.sep_token_id)
# [101, 7592, 2045, 1010, 2236, 6358, 16429, 2072, 999, 102]
# 101 102
```

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/1586?src=pr&el=h1) Report
> Merging [#1586](https://codecov.io/gh/huggingface/transformers/pull/1586?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/82f6abd98aaa691ca0adfe21e85a17dc6f386497?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

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```diff
@@           Coverage Diff           @@
##           master    #1586   +/-   ##
=======================================
  Coverage   86.16%   86.16%           
=======================================
  Files          91       91           
  Lines       13593    13593           
=======================================
  Hits        11713    11713           
  Misses       1880     1880
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/1586?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [transformers/modeling\_gpt2.py](https://codecov.io/gh/huggingface/transformers/pull/1586/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX2dwdDIucHk=) | `84.19% <ø> (ø)` | :arrow_up: |
| [transformers/modeling\_tf\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/1586/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX3RmX29wZW5haS5weQ==) | `96.04% <ø> (ø)` | :arrow_up: |
| [transformers/modeling\_tf\_transfo\_xl.py](https://codecov.io/gh/huggingface/transformers/pull/1586/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX3RmX3RyYW5zZm9feGwucHk=) | `92.21% <ø> (ø)` | :arrow_up: |
| [transformers/modeling\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/1586/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX3JvYmVydGEucHk=) | `80.57% <ø> (ø)` | :arrow_up: |
| [transformers/modeling\_transfo\_xl.py](https://codecov.io/gh/huggingface/transformers/pull/1586/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX3RyYW5zZm9feGwucHk=) | `75.16% <ø> (ø)` | :arrow_up: |
| [transformers/modeling\_tf\_ctrl.py](https://codecov.io/gh/huggingface/transformers/pull/1586/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX3RmX2N0cmwucHk=) | `97.75% <ø> (ø)` | :arrow_up: |
| [transformers/modeling\_xlm.py](https://codecov.io/gh/huggingface/transformers/pull/1586/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX3hsbS5weQ==) | `88.42% <ø> (ø)` | :arrow_up: |
| [transformers/modeling\_bert.py](https://codecov.io/gh/huggingface/transformers/pull/1586/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX2JlcnQucHk=) | `88.17% <ø> (ø)` | :arrow_up: |
| [transformers/modeling\_ctrl.py](https://codecov.io/gh/huggingface/transformers/pull/1586/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX2N0cmwucHk=) | `95.45% <ø> (ø)` | :arrow_up: |
| [transformers/modeling\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/1586/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX29wZW5haS5weQ==) | `81.75% <ø> (ø)` | :arrow_up: |
| ... and [7 more](https://codecov.io/gh/huggingface/transformers/pull/1586/diff?src=pr&el=tree-more) | |

------

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This is great thanks. Actually we should be adding this to all the examples for all the models...

@thomwolf  Would be happy to make the changes for the rest of the models.

@thomwolf added changes for the rest of the pytorch model examples and all the tensorflow model examples

I used the two bash scripts below to identify files to edit:
```
# Pytorch model examples
grep -iR "input_ids = torch.tensor(tokenizer.encode(" .

# Tensorflow model examples
grep -iR "input_ids = tf.constant(tokenizer.encode(" .
```

**UPDATE:** Example documentation changes were implemented for all tensorflow models except for ```modeling_tf_distilbert.py``` since ```TFDistilBertModelTest.test_pt_tf_model_equivalence``` would fail under **build_py3_torch_and_tf**  (details in error [logs](https://circleci.com/gh/huggingface/transformers/5245?utm_campaign=vcs-integration-link&utm_medium=referral&utm_source=github-build-link) for commit: ec276d6abad7eae800f1a1a039ddc78fde406009)

Thanks for that.

@LysandreJik even though we will have special tokens added by default in the coming release, maybe we still want to update the doc for the current release with this? (not sure this is possible)

I agree that the necessity to add special tokens should be explicit. However, the documentation is based on previous commits so changing the previous documentation would require to change the commit history of the repo (which we should not do). 

We might need to think of a way to work around that to update the misleading documentation of previous versions like in this case.