MbartTokenizer: do not hardcode vocab size
language codes should start at the end of the standard vocab. If the standard vocab is smaller, this number is not 250,001.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5998?src=pr&el=h1) Report
> Merging [#5998](https://codecov.io/gh/huggingface/transformers/pull/5998?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/33d7506ea10ca92886fd1bb3b5306a1a720c58fe&el=desc) will **increase** coverage by `0.00%`.
> The diff coverage is `100.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5998/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5998?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #5998   +/-   ##
=======================================
  Coverage   78.48%   78.49%           
=======================================
  Files         146      146           
  Lines       26230    26232    +2     
=======================================
+ Hits        20587    20591    +4     
+ Misses       5643     5641    -2     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5998?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_bart.py](https://codecov.io/gh/huggingface/transformers/pull/5998/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fYmFydC5weQ==) | `95.58% <100.00%> (+0.13%)` | :arrow_up: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5998/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `86.71% <0.00%> (+0.50%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/5998?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/5998?src=pr&el=footer). Last update [33d7506...39fdead](https://codecov.io/gh/huggingface/transformers/pull/5998?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
