Tokenization tutorial
This takes most of the description in #4510 and organizes it as a tokenizer tutorial for section 2 of the documentation.

Preview is [here](https://52645-155220641-gh.circle-artifacts.com/0/docs/_build/html/preprocessing.html).
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5257?src=pr&el=h1) Report
> Merging [#5257](https://codecov.io/gh/huggingface/transformers/pull/5257?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/0148c262e79f5ca12140d7fc35a6d3e0d80d5d3b&el=desc) will **decrease** coverage by `0.90%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5257/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5257?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5257      +/-   ##
==========================================
- Coverage   79.01%   78.10%   -0.91%     
==========================================
  Files         138      138              
  Lines       24064    24064              
==========================================
- Hits        19013    18795     -218     
- Misses       5051     5269     +218     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5257?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_t5.py](https://codecov.io/gh/huggingface/transformers/pull/5257/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl90NS5weQ==) | `49.30% <0.00%> (-42.12%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5257/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `85.71% <0.00%> (-2.07%)` | :arrow_down: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5257/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `91.14% <0.00%> (-0.13%)` | :arrow_down: |
| [src/transformers/trainer.py](https://codecov.io/gh/huggingface/transformers/pull/5257/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmVyLnB5) | `39.38% <0.00%> (+0.94%)` | :arrow_up: |
| [src/transformers/modeling\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/5257/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19vcGVuYWkucHk=) | `80.90% <0.00%> (+1.38%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/5257?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5257?src=pr&el=footer). Last update [0148c26...7bf3c46](https://codecov.io/gh/huggingface/transformers/pull/5257?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
