MarianTokenizer.prepare_translation_batch uses new tokenizer API
Author: sshleifer
Pull request: yes

new params:

```python
        truncation_strategy="only_first",
        padding="longest",
```
The old behavior was to pad to model_max_len then call `trim_batch`.

I also added two tests than ensure that it works.

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5182?src=pr&el=h1) Report
> Merging [#5182](https://codecov.io/gh/huggingface/transformers/pull/5182?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/c4d4e8bdbd25d9463d41de6398940329c89b7fb6&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `66.66%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5182/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5182?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5182      +/-   ##
==========================================
- Coverage   77.90%   77.88%   -0.02%     
==========================================
  Files         140      140              
  Lines       24334    24335       +1     
==========================================
- Hits        18957    18953       -4     
- Misses       5377     5382       +5     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5182?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_marian.py](https://codecov.io/gh/huggingface/transformers/pull/5182/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fbWFyaWFuLnB5) | `93.80% <66.66%> (+0.05%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_distilbert.py](https://codecov.io/gh/huggingface/transformers/pull/5182/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9kaXN0aWxiZXJ0LnB5) | `66.25% <0.00%> (-32.52%)` | :arrow_down: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5182/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `79.59% <0.00%> (+0.33%)` | :arrow_up: |
| [src/transformers/tokenization\_utils\_base.py](https://codecov.io/gh/huggingface/transformers/pull/5182/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHNfYmFzZS5weQ==) | `94.21% <0.00%> (+0.49%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_bert.py](https://codecov.io/gh/huggingface/transformers/pull/5182/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9iZXJ0LnB5) | `98.37% <0.00%> (+25.00%)` | :arrow_up: |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5182?src=pr&el=footer). Last update [c4d4e8b...5195adc](https://codecov.io/gh/huggingface/transformers/pull/5182?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).