Warning on `add_special_tokens`
Warning on `add_special_tokens` when passed to `encode`, `encode_plus` and `batch_encode_plus`
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2966?src=pr&el=h1) Report
> Merging [#2966](https://codecov.io/gh/huggingface/transformers/pull/2966?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/cc6775cdf5b20ad382613d3bdbf0dd8364d23219?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2966/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2966?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #2966      +/-   ##
==========================================
- Coverage   77.12%   77.11%   -0.01%     
==========================================
  Files          98       98              
  Lines       15977    15979       +2     
==========================================
  Hits        12322    12322              
- Misses       3655     3657       +2
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2966?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/2966/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHMucHk=) | `90.48% <100%> (+0.02%)` | :arrow_up: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/2966/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `92.2% <0%> (-0.33%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/2966?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/2966?src=pr&el=footer). Last update [cc6775c...7a09a54](https://codecov.io/gh/huggingface/transformers/pull/2966?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
