Doc tokenizer
Author: sgugger
Pull request: yes

Improve the documentation of tokenizers, following what was done for the models last week, mainly:
- make sure all docstrings of public functions are properly formatted for sphinx
- make sure all args are properly documented
- add or fix type hints wherever necessary

The methods/classes that are not in the main `__init__` are all in the page `internal/tokenization_utils.html`. I had added `SpecialTokensMixin` in the __init__ of transformers a while ago to easily document it, but it can be removed from there now if we want.

I rewrote a few dosctrings here and there so pinging @n1t0 and @mfuntowicz to make sure I didn't write anything bad.

[Preview](https://65719-155220641-gh.circle-artifacts.com/0/docs/_build/html/main_classes/tokenizer.html) of the tokenization page.
[Preview](https://65719-155220641-gh.circle-artifacts.com/0/docs/_build/html/internal/tokenization_utils.html) of the tokenization utils page.

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6110?src=pr&el=h1) Report
> Merging [#6110](https://codecov.io/gh/huggingface/transformers/pull/6110?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/11792d7826854979bb532b6da09bc3796b09ea6a&el=desc) will **decrease** coverage by `1.54%`.
> The diff coverage is `95.71%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/6110/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6110?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #6110      +/-   ##
==========================================
- Coverage   78.73%   77.19%   -1.55%     
==========================================
  Files         146      146              
  Lines       26314    26353      +39     
==========================================
- Hits        20719    20342     -377     
- Misses       5595     6011     +416     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/6110?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6110/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `87.32% <ø> (ø)` | |
| [src/transformers/data/data\_collator.py](https://codecov.io/gh/huggingface/transformers/pull/6110/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL2RhdGFfY29sbGF0b3IucHk=) | `97.39% <71.42%> (-1.71%)` | :arrow_down: |
| [src/transformers/tokenization\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6110/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHMucHk=) | `90.00% <88.88%> (+0.04%)` | :arrow_up: |
| [src/transformers/configuration\_encoder\_decoder.py](https://codecov.io/gh/huggingface/transformers/pull/6110/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX2VuY29kZXJfZGVjb2Rlci5weQ==) | `100.00% <100.00%> (ø)` | |
| [src/transformers/modeling\_bert.py](https://codecov.io/gh/huggingface/transformers/pull/6110/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19iZXJ0LnB5) | `88.39% <100.00%> (+0.01%)` | :arrow_up: |
| [src/transformers/modeling\_encoder\_decoder.py](https://codecov.io/gh/huggingface/transformers/pull/6110/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19lbmNvZGVyX2RlY29kZXIucHk=) | `92.20% <100.00%> (ø)` | |
| [src/transformers/modeling\_marian.py](https://codecov.io/gh/huggingface/transformers/pull/6110/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19tYXJpYW4ucHk=) | `90.90% <100.00%> (ø)` | |
| [src/transformers/modeling\_mobilebert.py](https://codecov.io/gh/huggingface/transformers/pull/6110/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19tb2JpbGViZXJ0LnB5) | `89.45% <100.00%> (+0.01%)` | :arrow_up: |
| [src/transformers/modeling\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/6110/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19vcGVuYWkucHk=) | `82.31% <100.00%> (+0.05%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_electra.py](https://codecov.io/gh/huggingface/transformers/pull/6110/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9lbGVjdHJhLnB5) | `95.53% <100.00%> (ø)` | |
| ... and [25 more](https://codecov.io/gh/huggingface/transformers/pull/6110/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/6110?src=pr&el=footer). Last update [11792d7...80a44a8](https://codecov.io/gh/huggingface/transformers/pull/6110?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).