Expose classes used in documentation
Author: sgugger
Pull request: yes

Currently, the documentation page of the tokenizers has three methods lacking documentation (see [here](https://huggingface.co/transformers/main_classes/tokenizer.html#pretrainedtokenizerfast)). This PR adds them to the `__init__` so sphynx can see them.

If there is one that should not be public, we should remove it from the documentation.

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4808?src=pr&el=h1) Report
> Merging [#4808](https://codecov.io/gh/huggingface/transformers/pull/4808?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/5c0cfc2cf0941d2db368767fd232d8712449c7f8&el=desc) will **increase** coverage by `0.39%`.
> The diff coverage is `100.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4808/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4808?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4808      +/-   ##
==========================================
+ Coverage   76.29%   76.69%   +0.39%     
==========================================
  Files         128      128              
  Lines       21495    21495              
==========================================
+ Hits        16400    16485      +85     
+ Misses       5095     5010      -85     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4808?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/\_\_init\_\_.py](https://codecov.io/gh/huggingface/transformers/pull/4808/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9fX2luaXRfXy5weQ==) | `99.13% <100.00%> (ø)` | |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4808/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `90.39% <0.00%> (+0.24%)` | :arrow_up: |
| [src/transformers/modeling\_bert.py](https://codecov.io/gh/huggingface/transformers/pull/4808/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19iZXJ0LnB5) | `89.17% <0.00%> (+2.01%)` | :arrow_up: |
| [src/transformers/modeling\_auto.py](https://codecov.io/gh/huggingface/transformers/pull/4808/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19hdXRvLnB5) | `78.40% <0.00%> (+4.80%)` | :arrow_up: |
| [src/transformers/configuration\_encoder\_decoder.py](https://codecov.io/gh/huggingface/transformers/pull/4808/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX2VuY29kZXJfZGVjb2Rlci5weQ==) | `100.00% <0.00%> (+61.53%)` | :arrow_up: |
| [src/transformers/modeling\_encoder\_decoder.py](https://codecov.io/gh/huggingface/transformers/pull/4808/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19lbmNvZGVyX2RlY29kZXIucHk=) | `92.20% <0.00%> (+64.93%)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4808?src=pr&el=footer). Last update [5c0cfc2...e2a7c2d](https://codecov.io/gh/huggingface/transformers/pull/4808?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).


Can we tell sphinx to look at more than just init? 

Looking at the sphinx documentation, there seems to be an option to use the modules and specify which parts of the module we want documented. Will try this as an alternative!

Looked further, but the workaround to use automodule and specifying a few functions will add the docstring of `tokenization_utils` and make the names longer (it becomes `transformers.tokenization_utils.SpecialTokensMixin` instead of `transformers.SpecialTokensMixin` which is fair enough, since it's not in transformers anymore).

Avoiding the module docstring seems possible by hacking something in conf.py but it then will be done globally, and may impact some other pages...

So merging this as is, and we can revisit if we really want to remove some of those things from `__init__`.