is_pretokenized -> is_split_into_words
As discussed offline, replacing `is_pretokenized` in `is_split_into_words` because the users were confused by what the argument represents.

Added deprecation warnings in the functions where the code ends, can add more if you feel I have forgotten some!
I think that’s a better name too!<|||||>@n1t0 We could also fix the name (with deprecation warnings) in tokenizers before the next release? Will then need three replacements in `tokenization_utils_fast.py`.<|||||># [Codecov](https://codecov.io/gh/huggingface/transformers/pull/7236?src=pr&el=h1) Report
> Merging [#7236](https://codecov.io/gh/huggingface/transformers/pull/7236?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/3a03bab9db342ff0e7104880326c68cc83bda010?el=desc) will **increase** coverage by `1.36%`.
> The diff coverage is `57.77%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/7236/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/7236?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #7236      +/-   ##
==========================================
+ Coverage   77.32%   78.68%   +1.36%     
==========================================
  Files         172      172              
  Lines       33089    33120      +31     
==========================================
+ Hits        25585    26061     +476     
+ Misses       7504     7059     -445     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/7236?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_utils\_base.py](https://codecov.io/gh/huggingface/transformers/pull/7236/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHNfYmFzZS5weQ==) | `94.04% <ø> (ø)` | |
| [src/transformers/tokenization\_utils\_fast.py](https://codecov.io/gh/huggingface/transformers/pull/7236/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHNfZmFzdC5weQ==) | `91.83% <42.85%> (-2.45%)` | :arrow_down: |
| [src/transformers/tokenization\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/7236/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHMucHk=) | `88.01% <56.25%> (-1.87%)` | :arrow_down: |
| [src/transformers/tokenization\_gpt2.py](https://codecov.io/gh/huggingface/transformers/pull/7236/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fZ3B0Mi5weQ==) | `93.50% <62.50%> (-3.72%)` | :arrow_down: |
| [src/transformers/tokenization\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/7236/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fcm9iZXJ0YS5weQ==) | `96.10% <66.66%> (-2.53%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_xlm.py](https://codecov.io/gh/huggingface/transformers/pull/7236/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl94bG0ucHk=) | `19.02% <0.00%> (-74.21%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_flaubert.py](https://codecov.io/gh/huggingface/transformers/pull/7236/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9mbGF1YmVydC5weQ==) | `24.53% <0.00%> (-63.81%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_gpt2.py](https://codecov.io/gh/huggingface/transformers/pull/7236/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9ncHQyLnB5) | `71.59% <0.00%> (-23.38%)` | :arrow_down: |
| [src/transformers/modeling\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/7236/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19vcGVuYWkucHk=) | `72.25% <0.00%> (-10.00%)` | :arrow_down: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/7236/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `81.70% <0.00%> (-4.77%)` | :arrow_down: |
| ... and [13 more](https://codecov.io/gh/huggingface/transformers/pull/7236/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/7236?src=pr&el=footer). Last update [3a03bab...489bb9a](https://codecov.io/gh/huggingface/transformers/pull/7236?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
