Text classification pipeline outputs differ with 4.20
### System Info

```shell
transformers 4.20.0
```


### Who can help?

@Narsil

### Information

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

### Tasks

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

### Reproduction

# 4.19.4

```python
from transformers import pipeline

nlp = pipeline("text-classification")
nlp("happy", return_all_scores=True)
```

Output:
```
[[{'label': 'NEGATIVE', 'score': 0.0001246821484528482}, {'label': 'POSITIVE', 'score': 0.9998753070831299}]]
```

# 4.20.0
```python
from transformers import pipeline

nlp = pipeline("text-classification")
nlp("happy", return_all_scores=True)
```

Output:
```
[{'label': 'NEGATIVE', 'score': 0.0001246821484528482}, {'label': 'POSITIVE', 'score': 0.9998753070831299}]
```

Running with top_k=None also produces a single list (only difference is labels are ordered by score desc).

```python
from transformers import pipeline

nlp = pipeline("text-classification")
nlp("happy", top_k=None)
```

Output:
```
[{'label': 'POSITIVE', 'score': 0.9998753070831299}, {'label': 'NEGATIVE', 'score': 0.0001246821484528482}]
```

4.19.4 returns a list of lists when return_all_scores=True. 4.20.0 only produces a single list.

It looks like this logic changed the outputs in the `__call__` method.

```python
        if isinstance(args[0], str) and isinstance(result, dict):
            # This pipeline is odd, and return a list when single item is run
            return [result]
        else:
            return result
```

Previously, it was this:

```python
        if isinstance(args[0], str):
            # This pipeline is odd, and return a list when single item is run
            return [result]
        else:
            return result
```

### Expected behavior

```shell
When passing a single text element, the pipeline up to 4.20 would return a list. If this change was expected, I can work with it but figured it was worth bringing to your attention in case it wasn't intentional.
```

Just wanted to check in on this and see if it's considered an issue or the new way the pipeline works. <|||||>@davidmezzetti Didn't see this issue, but I didn't see the regression.

Should have been fixed here https://github.com/huggingface/transformers/pull/17906. Sorry it had time to ship with `4.20`. It will be reverted back in the next release.

We are really keen to not break anything format wise while we are in `v4`. But for `v5` harmonizing return types of pipelines is definitely on the agenda I want to push (some return lists, lists of lists, but we're not super consistent across pipelines.).

<|||||>Thank you for responding and the update!<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md) are likely to be ignored.