Tokenizer.batch_decode convenience method
convenience method that turns the list returned by `model.generate` into a list of sentences.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4159?src=pr&el=h1) Report
> Merging [#4159](https://codecov.io/gh/huggingface/transformers/pull/4159?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/7822cd38a0e18004ab1a55bfe85e8b3bc0d8857a&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/4159/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4159?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #4159   +/-   ##
=======================================
  Coverage   78.14%   78.15%           
=======================================
  Files         120      120           
  Lines       20053    20053           
=======================================
+ Hits        15670    15672    +2     
+ Misses       4383     4381    -2     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4159?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_marian.py](https://codecov.io/gh/huggingface/transformers/pull/4159/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fbWFyaWFuLnB5) | `87.32% <ø> (-0.35%)` | :arrow_down: |
| [src/transformers/tokenization\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4159/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHMucHk=) | `89.51% <100.00%> (+0.02%)` | :arrow_up: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4159/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `90.60% <0.00%> (+0.12%)` | :arrow_up: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4159/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `73.85% <0.00%> (+0.41%)` | :arrow_up: |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4159?src=pr&el=footer). Last update [7822cd3...cc21682](https://codecov.io/gh/huggingface/transformers/pull/4159?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>I'm in favor of this since we also have a `batch_encode_plus` method. <|||||>One small thing: the `encode_plus` method uses the `batch_encode_plus` whereas here it's the other way around. But this does not bother me too much.<|||||>I don't have a strong knowledge of the API design for tokenizers so will let others chime in instead.
