[s2s]Use prepare_translation_batch for Marian finetuning
Fix #6262 
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6293?src=pr&el=h1) Report
> Merging [#6293](https://codecov.io/gh/huggingface/transformers/pull/6293?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/d5bc32ce92ace9aaec7752e0b89d51ba18903a1b&el=desc) will **decrease** coverage by `0.55%`.
> The diff coverage is `0.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/6293/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6293?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #6293      +/-   ##
==========================================
- Coverage   79.56%   79.00%   -0.56%     
==========================================
  Files         147      147              
  Lines       27125    27127       +2     
==========================================
- Hits        21581    21433     -148     
- Misses       5544     5694     +150     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/6293?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_marian.py](https://codecov.io/gh/huggingface/transformers/pull/6293/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fbWFyaWFuLnB5) | `66.95% <0.00%> (-26.85%)` | :arrow_down: |
| [src/transformers/tokenization\_xlm.py](https://codecov.io/gh/huggingface/transformers/pull/6293/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25feGxtLnB5) | `16.26% <0.00%> (-66.67%)` | :arrow_down: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6293/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `82.44% <0.00%> (+0.25%)` | :arrow_up: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6293/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `86.46% <0.00%> (+5.76%)` | :arrow_up: |
| [src/transformers/tokenization\_bart.py](https://codecov.io/gh/huggingface/transformers/pull/6293/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fYmFydC5weQ==) | `95.77% <0.00%> (+35.21%)` | :arrow_up: |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6293?src=pr&el=footer). Last update [d5bc32c...ea610f8](https://codecov.io/gh/huggingface/transformers/pull/6293?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>thanks for the nit!