Fix marian tokenizer save pretrained
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5043?src=pr&el=h1) Report
> Merging [#5043](https://codecov.io/gh/huggingface/transformers/pull/5043?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/36434220fc807c5015bc8f0f1e50ab21f7d34914&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/5043/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5043?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #5043   +/-   ##
=======================================
  Coverage   77.36%   77.37%           
=======================================
  Files         130      130           
  Lines       21989    21990    +1     
=======================================
+ Hits        17012    17014    +2     
+ Misses       4977     4976    -1     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5043?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_marian.py](https://codecov.io/gh/huggingface/transformers/pull/5043/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fbWFyaWFuLnB5) | `92.85% <100.00%> (+0.96%)` | :arrow_up: |

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