[docs] Add integration test example to copy pasta template
Author: sshleifer
Pull request: yes

Encourage testing practices that have been encouraged since last update, namely:

- @slow tests that run on cuda and fp16 if possible and show that your model produces good outputs.
- more tests ~= better
- unindent the ModelTester
- call get_extended_attention_mask and delete massive comment.

My code is probably broken because this thing isn't tested!


Add:
"""
Try to make this test take a string and check that a resultant string == desired_result using your tokenizers encode and decode functions
""

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5961?src=pr&el=h1) Report
> Merging [#5961](https://codecov.io/gh/huggingface/transformers/pull/5961?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/e714412fe6b38346a1f73525b701e030857b2f21&el=desc) will **decrease** coverage by `1.22%`.
> The diff coverage is `25.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5961/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5961?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5961      +/-   ##
==========================================
- Coverage   78.50%   77.27%   -1.23%     
==========================================
  Files         146      146              
  Lines       26214    26218       +4     
==========================================
- Hits        20578    20259     -319     
- Misses       5636     5959     +323     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5961?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/testing\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5961/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90ZXN0aW5nX3V0aWxzLnB5) | `72.72% <25.00%> (-3.75%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_mobilebert.py](https://codecov.io/gh/huggingface/transformers/pull/5961/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9tb2JpbGViZXJ0LnB5) | `23.38% <0.00%> (-73.39%)` | :arrow_down: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5961/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `83.95% <0.00%> (-1.26%)` | :arrow_down: |
| [src/transformers/generation\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5961/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3V0aWxzLnB5) | `96.82% <0.00%> (-0.29%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/5961/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9yb2JlcnRhLnB5) | `93.36% <0.00%> (+49.37%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/5961?src=pr&el=continue).
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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5961?src=pr&el=footer). Last update [e714412...a57f645](https://codecov.io/gh/huggingface/transformers/pull/5961?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).