Documentation error 
## 

Hi, I think there is a tiny issue in the documentation of using pipelines

The current [doc](https://github.com/huggingface/transformers/blob/4a6a35bc6506dd5deb528580184c39e3e778d53b/docs/source/main_classes/pipelines.mdx) says that the KeyDataset class is in the transformers.pipelines.base, however the class is actually in the [pt_utils](https://github.com/huggingface/transformers/blob/19d37c2dd36d73537a6855c56808e524fb584459/src/transformers/pipelines/pt_utils.py)


```python
#The current doc (not working)
from transformers.pipelines.base import KeyDataset
```

```python
#The actual class is in 
from transformers.pipelines.pt_utils import KeyDataset
```

Running the current doc's examples won't work : 
```
ImportError: cannot import name 'KeyDataset' from 'transformers.pipelines.base' (/usr/local/lib/python3.7/dist-packages/transformers/pipelines/base.py)
```
Hi,

Thanks for spotting. Feel free to open a PR to fix this!<|||||>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/master/CONTRIBUTING.md) are likely to be ignored.