Adding PaddingDataCollator
New version of #6398 
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6442?src=pr&el=h1) Report
> Merging [#6442](https://codecov.io/gh/huggingface/transformers/pull/6442?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/96c3329f19f28e47eab7f9f20ed3504619e16722&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `52.94%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/6442/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6442?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #6442      +/-   ##
==========================================
- Coverage   79.95%   79.93%   -0.02%     
==========================================
  Files         153      153              
  Lines       27932    27947      +15     
==========================================
+ Hits        22332    22339       +7     
- Misses       5600     5608       +8     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/6442?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/\_\_init\_\_.py](https://codecov.io/gh/huggingface/transformers/pull/6442/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9fX2luaXRfXy5weQ==) | `99.27% <ø> (ø)` | |
| [src/transformers/data/data\_collator.py](https://codecov.io/gh/huggingface/transformers/pull/6442/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL2RhdGFfY29sbGF0b3IucHk=) | `90.90% <52.94%> (-5.68%)` | :arrow_down: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6442?src=pr&el=footer). Last update [96c3329...a153ed4](https://codecov.io/gh/huggingface/transformers/pull/6442?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
