 'Speech2TextProcessor' has no attribute 'from_pretrained'`
## Environment info
<!-- You can run the command `transformers-cli env` and copy-and-paste its output below.
     Don't forget to fill out the missing fields in that output! -->

- `transformers` version: 4.6.1
- Platform: ubuntu
- Python version: 3.7.10
- PyTorch version (GPU?): cpu 1.8.1
- Tensorflow version (GPU?): no tensorflow
- Using GPU in script?: no cpu



Models:

Speech2TextProcessor

## To reproduce

Steps to reproduce the behavior:

1. processor =Speech2TextProcessor.from_pretrained("facebook/s2t-small-librispeech-asr")



The error I got : 

`AttributeError: type object 'Speech2TextProcessor' has no attribute 'from_pretrained'`

<!-- A clear and concise description of what you would expect to happen. -->

I don't understand why I got this issue because in processing_speech_to_text.py, line 78 "from_pretrained" exist.
Hello! I'm fixing this in #12145 to return a better error.