Inconsistency multiple mask in fill-mask
### System Info

```shell
- `transformers` version: 4.14.1
- Platform: Darwin-21.5.0-x86_64-i386-64bit
- Python version: 3.6.13
- PyTorch version (GPU?): 1.10.2 (False)
- Tensorflow version (GPU?): not installed (NA)
- Flax version (CPU?/GPU?/TPU?): not installed (NA)
- Jax version: not installed
- JaxLib version: not installed
- Using GPU in script?: no
- Using distributed or parallel set-up in script?: no
```


### Who can help?

@Narsil

### Information

- [ ] The official example scripts
- [X] My own modified scripts

### Tasks

- [X] An officially supported task in the `examples` folder (such as GLUE/SQuAD, ...)
- [ ] My own task or dataset (give details below)

### Reproduction

```
pipe = pipeline("fill-mask", top_k=1)

pipe("It is <mask>.")
=> [{..., 'sequence': 'It is true.'}]

pipe("It is <mask> <mask>.")
=> [[{..., 'sequence': '<s>It is very<mask>.</s>'}], [{..., 'sequence': '<s>It is<mask>orable.</s>'}]]
```

### Expected behavior


I would expect that the "sequence"  does not include `<s>` and `</s>` tokens. It also seems to remove whitespace before the `<mask>` tokens left in the result, but I believe that does not make a difference for the tokenizer.

What I would expect:
```shell
pipe = pipeline("fill-mask", top_k=1)

pipe("It is <mask>.")
=> [{..., 'sequence': 'It is true.'}]

pipe("It is <mask><mask>.")
=> [[{..., 'sequence': 'It is very <mask>.'}], [{..., 'sequence': 'It is <mask>orable.'}]]
```

nvm it seems deliberate  https://github.com/huggingface/transformers/blob/374a2f693f75305eded1a2bb7a7e452f0ab33fad/src/transformers/pipelines/fill_mask.py#L137-L140