Fix doc examples: cannot import name
# What does this PR do?

In doc examples in some PT model files, there are `cannot import name` error. This PR fixes this issue.

## Who can review?

@patrickvonplaten 

## Question:

@patrickvonplaten , in `modeling_unispeech.py`, there are examples using

```
from transformers import UniSpeechSatFeatureExtractor
```
And in some docstring, there are 
```
:class:`~transformers.UniSpeechSatProcessor`
```

However, these can't be imported

```
ImportError: cannot import name 'UniSpeechSatFeatureExtractor' from 'transformers' (/home/ydshieh/Desktop/ydshieh/transformers/src/transformers/__init__.py)
```
Is there any reason to make these objects invisible to `transformers`?
Great catch - thanks a lot for fixing it! I think we should do the exact same thing for the speech models (UniSpeech, SEW, UniSpeechSAT) actually. Feel free to open a PR if you want - I'll put it on my ToDo list as well though<|||||>@patrickvonplaten 

So `UniSpeechSatFeatureExtractor` shouldn't be used by `UniSpeechSatModel` , and it is supposed to use `Wav2Vec2FeatureExtractor`?

I have seen 

```
_PROCESSOR_FOR_DOC = "Wav2Vec2Processor"
_SEQ_CLASS_PROCESSOR_FOR_DOC = "Wav2Vec2FeatureExtractor"
```
and `UniSpeechSatFeatureExtractor` is not visible to `transformers`. But I don't have the full context.<|||||>> @patrickvonplaten
> 
> So `UniSpeechSatFeatureExtractor` shouldn't be used by `UniSpeechSatModel` , and it is supposed to use `Wav2Vec2FeatureExtractor`?
> 
> I have seen
> 
> ```
> _PROCESSOR_FOR_DOC = "Wav2Vec2Processor"
> _SEQ_CLASS_PROCESSOR_FOR_DOC = "Wav2Vec2FeatureExtractor"
> ```
> 
> and `UniSpeechSatFeatureExtractor` is not visible to `transformers`. But I don't have the full context.

Yeah there is no `UniSpeechFeatureExtractor` since it's the same as `Wav2Vec2FeatureExtractor` so we should instead just use `Wav2Vec2FeatureExtractor`. Happy to resolve this is another PR though!<|||||>@ydshieh - if ok for you, I would like to merge this one now and then if you want we could open a new one for `UniSpeechFeatureExtractor`<|||||>> @ydshieh - if ok for you, I would like to merge this one now and then if you want we could open a new one for `UniSpeechFeatureExtractor`

Sure, ok for me.