Add `return_special_tokens_mask` to `batch_encode_plus()`
Proposal to add the keyword argument `return_special_tokens_mask` to the method `batch_encode_plus()` to match the functionality of `encode_plus()`. The implementation simply adds the argument in the `encode_plus()` call, so it inherits its implementation and should be compatible with other changes to the `batch_encode_plus()` arguments.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2657?src=pr&el=h1) Report
> Merging [#2657](https://codecov.io/gh/huggingface/transformers/pull/2657?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/875c4ae48f97af9792ab0b87b49a426ca7e7586b?src=pr&el=desc) will **decrease** coverage by `1.1%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2657/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2657?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #2657      +/-   ##
==========================================
- Coverage   74.58%   73.47%   -1.11%     
==========================================
  Files          87       87              
  Lines       14892    14892              
==========================================
- Hits        11107    10942     -165     
- Misses       3785     3950     +165
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2657?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/2657/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHMucHk=) | `85.69% <ø> (ø)` | :arrow_up: |
| [src/transformers/modeling\_tf\_pytorch\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/2657/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9weXRvcmNoX3V0aWxzLnB5) | `8.72% <0%> (-81.21%)` | :arrow_down: |
| [src/transformers/modeling\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/2657/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19yb2JlcnRhLnB5) | `55.39% <0%> (-9.86%)` | :arrow_down: |
| [src/transformers/modeling\_xlnet.py](https://codecov.io/gh/huggingface/transformers/pull/2657/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ194bG5ldC5weQ==) | `70.94% <0%> (-2.28%)` | :arrow_down: |
| [src/transformers/modeling\_ctrl.py](https://codecov.io/gh/huggingface/transformers/pull/2657/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19jdHJsLnB5) | `92.07% <0%> (-2.21%)` | :arrow_down: |
| [src/transformers/modeling\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/2657/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19vcGVuYWkucHk=) | `80.06% <0%> (-1.33%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/2657?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2657?src=pr&el=footer). Last update [875c4ae...1f3e2b6](https://codecov.io/gh/huggingface/transformers/pull/2657?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.
