Have dummy processors have a `from_pretrained` method
Fix https://github.com/huggingface/transformers/issues/12100

Before:

```py
>>> from transformers import Speech2TextProcessor
>>> processor =Speech2TextProcessor.from_pretrained("facebook/s2t-small-librispeech-asr")
```
```out
Traceback (most recent call last):
  File "<input>", line 1, in <module>
AttributeError: type object 'Speech2TextProcessor' has no attribute 'from_pretrained'
```

After:

```py
>>> from transformers import Speech2TextProcessor
>>> processor =Speech2TextProcessor.from_pretrained("facebook/s2t-small-librispeech-asr")
```
```out
Traceback (most recent call last):
  File "<input>", line 1, in <module>
  File "/home/lysandre/Workspaces/Python/transformers/src/transformers/utils/dummy_sentencepiece_and_speech_objects.py", line 11, in from_pretrained
    requires_backends(cls, ["sentencepiece", "speech"])
  File "/home/lysandre/Workspaces/Python/transformers/src/transformers/file_utils.py", line 606, in requires_backends
    raise ImportError("".join([BACKENDS_MAPPING[backend][1].format(name) for backend in backends]))
ImportError: 
Speech2TextProcessor requires the SentencePiece library but it was not found in your environment. Checkout the instructions on the
installation page of its repo: https://github.com/google/sentencepiece#installation and follow the ones
that match your environment.

Speech2TextProcessor requires the torchaudio library but it was not found in your environment. You can install it with pip:
`pip install torchaudio`


```