[pipelines] Update fill mask pipeline to remove special tokens in the output
Small fix to remove the special tokens from the output of the fill mask pipeline.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5681?src=pr&el=h1) Report
> Merging [#5681](https://codecov.io/gh/huggingface/transformers/pull/5681?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/7fad617dc1fc681a7f5da5e0172c8b83f4bf0024&el=desc) will **decrease** coverage by `0.20%`.
> The diff coverage is `100.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5681/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5681?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5681      +/-   ##
==========================================
- Coverage   78.11%   77.91%   -0.21%     
==========================================
  Files         146      146              
  Lines       25983    25983              
==========================================
- Hits        20297    20244      -53     
- Misses       5686     5739      +53     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5681?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/pipelines.py](https://codecov.io/gh/huggingface/transformers/pull/5681/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9waXBlbGluZXMucHk=) | `76.36% <100.00%> (ø)` | |
| [src/transformers/modeling\_tf\_t5.py](https://codecov.io/gh/huggingface/transformers/pull/5681/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl90NS5weQ==) | `44.56% <0.00%> (-46.35%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_gpt2.py](https://codecov.io/gh/huggingface/transformers/pull/5681/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9ncHQyLnB5) | `63.55% <0.00%> (-31.78%)` | :arrow_down: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5681/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `79.44% <0.00%> (-6.52%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5681/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `86.92% <0.00%> (-1.97%)` | :arrow_down: |
| [src/transformers/modeling\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/5681/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19vcGVuYWkucHk=) | `82.31% <0.00%> (+1.28%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/5681/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9yb2JlcnRhLnB5) | `93.36% <0.00%> (+49.37%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/5681/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9vcGVuYWkucHk=) | `95.18% <0.00%> (+74.91%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/5681?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/5681?src=pr&el=footer). Last update [7fad617...c2fbb71](https://codecov.io/gh/huggingface/transformers/pull/5681?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Hi Thomas, I found the output is something like this with the latest version of `transformers`:
```json
[
  {
    "sequence": "<s>if (x is not None) and(x>1)</s>",
    "score": 0.7236990928649902,
    "token": 8,
    "token_str": "Ġand"
  },
  {
    "sequence": "<s>if (x is not None) &(x>1)</s>",
    "score": 0.10633797943592072,
    "token": 359,
    "token_str": "Ġ&"
  },
  {
    "sequence": "<s>if (x is not None)and(x>1)</s>",
    "score": 0.021604137495160103,
    "token": 463,
    "token_str": "and"
  },
  {
    "sequence": "<s>if (x is not None) AND(x>1)</s>",
    "score": 0.02122747339308262,
    "token": 4248,
    "token_str": "ĠAND"
  },
  {
    "sequence": "<s>if (x is not None) if(x>1)</s>",
    "score": 0.016991324722766876,
    "token": 114,
    "token_str": "Ġif"
  }
]
```

However, when using `2.9.1`, I get:
```python
{'sequence': '<s> if (x is not None) and (x>1)</s>', 'score': 0.6049249172210693, 'token': 8}
{'sequence': '<s> if (x is not None) or (x>1)</s>', 'score': 0.30680200457572937, 'token': 50}
{'sequence': '<s> if (x is not None) if (x>1)</s>', 'score': 0.02133703976869583, 'token': 114}
{'sequence': '<s> if (x is not None) then (x>1)</s>', 'score': 0.018607674166560173, 'token': 172}
{'sequence': '<s> if (x is not None) AND (x>1)</s>', 'score': 0.007619690150022507, 'token': 4248}
```

The output sequence of `2.9.1` is way much cleaner. Can this PR fix this?<|||||>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.
