[FIX] #2658 Inconsistent values returned by batch_encode_plus and enc…
As ticket describe, when using batch_encode_plus, instead of encode_plus, tokens type and mask are different. They should be the same using batch processing or not. Proposed fix here solve the issue
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2659?src=pr&el=h1) Report
> Merging [#2659](https://codecov.io/gh/huggingface/transformers/pull/2659?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/e0849a66accda8aa435a3db164c373175115a5b0?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2659/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2659?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #2659      +/-   ##
==========================================
+ Coverage   74.51%   74.51%   +<.01%     
==========================================
  Files          87       87              
  Lines       14920    14920              
==========================================
+ Hits        11117    11118       +1     
+ Misses       3803     3802       -1
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2659?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/2659/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHMucHk=) | `85.85% <ø> (+0.16%)` | :arrow_up: |
| [src/transformers/commands/serving.py](https://codecov.io/gh/huggingface/transformers/pull/2659/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb21tYW5kcy9zZXJ2aW5nLnB5) | `0% <ø> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/2659?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2659?src=pr&el=footer). Last update [e0849a6...6efcbec](https://codecov.io/gh/huggingface/transformers/pull/2659?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
