Raises PipelineException on FillMaskPipeline when there are != 1 mask_token in the input
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5389?src=pr&el=h1) Report
> Merging [#5389](https://codecov.io/gh/huggingface/transformers/pull/5389?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/64e3d966b1131c15b5905b1e1e582d4bebac1ef0&el=desc) will **decrease** coverage by `0.16%`.
> The diff coverage is `100.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5389/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5389?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5389      +/-   ##
==========================================
- Coverage   77.75%   77.59%   -0.17%     
==========================================
  Files         140      140              
  Lines       24373    24384      +11     
==========================================
- Hits        18951    18920      -31     
- Misses       5422     5464      +42     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5389?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/pipelines.py](https://codecov.io/gh/huggingface/transformers/pull/5389/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9waXBlbGluZXMucHk=) | `75.79% <100.00%> (+0.47%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_electra.py](https://codecov.io/gh/huggingface/transformers/pull/5389/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9lbGVjdHJhLnB5) | `26.92% <0.00%> (-68.47%)` | :arrow_down: |
| [src/transformers/tokenization\_auto.py](https://codecov.io/gh/huggingface/transformers/pull/5389/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fYXV0by5weQ==) | `95.55% <0.00%> (-2.23%)` | :arrow_down: |
| [src/transformers/configuration\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5389/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX3V0aWxzLnB5) | `95.68% <0.00%> (-0.72%)` | :arrow_down: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5389/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `79.26% <0.00%> (-0.34%)` | :arrow_down: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5389/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `86.43% <0.00%> (+0.75%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_pytorch\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5389/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9weXRvcmNoX3V0aWxzLnB5) | `89.30% <0.00%> (+1.25%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5389/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `89.11% <0.00%> (+5.10%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_gpt2.py](https://codecov.io/gh/huggingface/transformers/pull/5389/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9ncHQyLnB5) | `95.14% <0.00%> (+29.44%)` | :arrow_up: |
| [src/transformers/tokenization\_xlm\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/5389/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25feGxtX3JvYmVydGEucHk=) | `95.23% <0.00%> (+33.33%)` | :arrow_up: |

------

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<|||||>no particular reason @LysandreJik other than developer lazyness (me)<|||||>> no particular reason @LysandreJik other than developer lazyness (me)

@julien-c 

Hi Julien. I understand if the implementation of multiple tokens is too inconvenient, but I am sure that many would appreciate such a feature. <|||||>Hey @mfuntowicz , @julien-c,

> LGTM! Why don't we handle multiple tokens, by the way? The models should behave somewhat okay given that their pre-training hides more than one token, right?

I am implementing this feature in my local version, is there anything I should be careful of? I am just planning to remove the exception trigger and handle the multiple masked tokens output. 

Also, let me know if you'd like a PR for this. 
