[Fix] seq2seq pack_dataset.py actually packs
Added stronger test (that failed before small code fixes).

# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5913?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@f197511`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5913/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5913?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master    #5913   +/-   ##
=========================================
  Coverage          ?   78.69%           
=========================================
  Files             ?      146           
  Lines             ?    26214           
  Branches          ?        0           
=========================================
  Hits              ?    20629           
  Misses            ?     5585           
  Partials          ?        0           
```



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