[tokenizers] Updates data processors, docstring, examples and model cards to the new API
Updates the data-processors to the new recommended tokenizers' API instead of the old one.
Also update the docstrings, examples, and model-cards which were using the old API.

Supersede #5310

@sshleifer you have a couple of methods only your models use (bart and marian). I'm not sure about the consequences of updating those API so I'll let you update them. Here is the doc on the new tokenizer API if you need it: https://huggingface.co/transformers/master/preprocessing.html
Recommended updates:
- use `__call__` instead of `encode_plus` and `batch_encode_plus`
- use `padding` and `truncation` instead of `max_length` only and `pad_to_max_length`
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5308?src=pr&el=h1) Report
> Merging [#5308](https://codecov.io/gh/huggingface/transformers/pull/5308?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/135791e8ef12802ceb21a4abbb3a93f7da1bf390&el=desc) will **decrease** coverage by `2.10%`.
> The diff coverage is `58.82%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5308/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5308?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5308      +/-   ##
==========================================
- Coverage   79.30%   77.20%   -2.11%     
==========================================
  Files         138      138              
  Lines       24283    24285       +2     
==========================================
- Hits        19258    18749     -509     
- Misses       5025     5536     +511     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5308?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/data/processors/squad.py](https://codecov.io/gh/huggingface/transformers/pull/5308/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL3Byb2Nlc3NvcnMvc3F1YWQucHk=) | `28.66% <0.00%> (ø)` | |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5308/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `79.59% <ø> (ø)` | |
| [src/transformers/modeling\_albert.py](https://codecov.io/gh/huggingface/transformers/pull/5308/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19hbGJlcnQucHk=) | `80.73% <ø> (ø)` | |
| [src/transformers/modeling\_bert.py](https://codecov.io/gh/huggingface/transformers/pull/5308/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19iZXJ0LnB5) | `87.65% <ø> (ø)` | |
| [src/transformers/modeling\_ctrl.py](https://codecov.io/gh/huggingface/transformers/pull/5308/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19jdHJsLnB5) | `96.62% <ø> (-2.54%)` | :arrow_down: |
| [src/transformers/modeling\_distilbert.py](https://codecov.io/gh/huggingface/transformers/pull/5308/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19kaXN0aWxiZXJ0LnB5) | `96.04% <ø> (-1.70%)` | :arrow_down: |
| [src/transformers/modeling\_electra.py](https://codecov.io/gh/huggingface/transformers/pull/5308/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19lbGVjdHJhLnB5) | `80.62% <ø> (ø)` | |
| [src/transformers/modeling\_flaubert.py](https://codecov.io/gh/huggingface/transformers/pull/5308/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19mbGF1YmVydC5weQ==) | `84.37% <ø> (ø)` | |
| [src/transformers/modeling\_gpt2.py](https://codecov.io/gh/huggingface/transformers/pull/5308/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19ncHQyLnB5) | `85.94% <ø> (ø)` | |
| [src/transformers/modeling\_longformer.py](https://codecov.io/gh/huggingface/transformers/pull/5308/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19sb25nZm9ybWVyLnB5) | `93.11% <ø> (ø)` | |
| ... and [47 more](https://codecov.io/gh/huggingface/transformers/pull/5308/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/5308?src=pr&el=footer). Last update [135791e...5b1fbb2](https://codecov.io/gh/huggingface/transformers/pull/5308?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>@sshleifer @patrickvonplaten and @yjernite I updated your examples (seq2seq and eli5). Maybe you want to check it.<|||||>> @sshleifer @patrickvonplaten and @yjernite I updated your examples (seq2seq and eli5). Maybe you want to check it.

lgtm!