Tokenizer summary
This PR introduces a mid/high-level summary of the different tokenizer types used in the library (a bit like the model summary).

Preview is [here](https://56179-155220641-gh.circle-artifacts.com/0/docs/_build/html/tokenizer_summary.html).
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5467?src=pr&el=h1) Report
> Merging [#5467](https://codecov.io/gh/huggingface/transformers/pull/5467?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/35befd9ce31c23a774fd34f57bc44033ce70141d&el=desc) will **decrease** coverage by `0.08%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5467/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5467?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5467      +/-   ##
==========================================
- Coverage   77.57%   77.48%   -0.09%     
==========================================
  Files         141      141              
  Lines       24581    24581              
==========================================
- Hits        19068    19046      -22     
- Misses       5513     5535      +22     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5467?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_xlm\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/5467/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25feGxtX3JvYmVydGEucHk=) | `61.90% <0.00%> (-33.34%)` | :arrow_down: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5467/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `85.42% <0.00%> (+1.50%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/5467?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5467?src=pr&el=footer). Last update [35befd9...80529fa](https://codecov.io/gh/huggingface/transformers/pull/5467?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
