rename prepare_translation_batch -> prepare_seq2seq_batch
Author: sshleifer
Pull request: yes

cc @patil-suraj 

Starts work on #6080 , which suggests that all seq2seq tokenizers expose a `prepare_seq2seq_batch` method.


TODO:
- add common test enforcing API consistency.

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6103?src=pr&el=h1) Report
> Merging [#6103](https://codecov.io/gh/huggingface/transformers/pull/6103?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/66fa8ceaeaa6fe12f1bd4a5e6b0a924f59f715d9&el=desc) will **decrease** coverage by `0.47%`.
> The diff coverage is `100.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/6103/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6103?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #6103      +/-   ##
==========================================
- Coverage   79.90%   79.42%   -0.48%     
==========================================
  Files         153      153              
  Lines       27877    27879       +2     
==========================================
- Hits        22276    22144     -132     
- Misses       5601     5735     +134     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/6103?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_marian.py](https://codecov.io/gh/huggingface/transformers/pull/6103/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19tYXJpYW4ucHk=) | `90.00% <ø> (ø)` | |
| [src/transformers/tokenization\_bart.py](https://codecov.io/gh/huggingface/transformers/pull/6103/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fYmFydC5weQ==) | `95.77% <100.00%> (ø)` | |
| [src/transformers/tokenization\_marian.py](https://codecov.io/gh/huggingface/transformers/pull/6103/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fbWFyaWFuLnB5) | `99.14% <100.00%> (+6.97%)` | :arrow_up: |
| [src/transformers/tokenization\_xlm.py](https://codecov.io/gh/huggingface/transformers/pull/6103/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25feGxtLnB5) | `16.26% <0.00%> (-66.67%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_flaubert.py](https://codecov.io/gh/huggingface/transformers/pull/6103/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9mbGF1YmVydC5weQ==) | `24.53% <0.00%> (-63.20%)` | :arrow_down: |
| [src/transformers/tokenization\_ctrl.py](https://codecov.io/gh/huggingface/transformers/pull/6103/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fY3RybC5weQ==) | `78.64% <0.00%> (-17.48%)` | :arrow_down: |
| [src/transformers/tokenization\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/6103/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fb3BlbmFpLnB5) | `71.21% <0.00%> (-12.88%)` | :arrow_down: |
| [src/transformers/tokenization\_transfo\_xl.py](https://codecov.io/gh/huggingface/transformers/pull/6103/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdHJhbnNmb194bC5weQ==) | `33.56% <0.00%> (-8.93%)` | :arrow_down: |
| [src/transformers/tokenization\_pegasus.py](https://codecov.io/gh/huggingface/transformers/pull/6103/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fcGVnYXN1cy5weQ==) | `95.16% <0.00%> (+1.61%)` | :arrow_up: |
| ... and [2 more](https://codecov.io/gh/huggingface/transformers/pull/6103/diff?src=pr&el=tree-more) | |

------

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Didn't know that `prepare_translation_batch` was already in master. Guess I'm not a huge fan of such a helper function in general and for me, it's a pure convenience function that does not really add functionality to the lib.  Think we should lower maintenance costs and reduce the risk of future breaking backward compatibility by not adding such functions to the python tokenizers. But I don't have the best insight into the tokenizers. Maybe @LysandreJik and @thomwolf can have a better opinion here.

The better argument is not about convenience, but about managing special tokens when they are different on the encoder and decoder side.  It's very hard to have finetuning code that supports multiple models if the tokenizers don't handle special tokens/language codes for you. 



same spurious pabee failure as #6421 , merging!