Allow creation of tokenizer from a vocab dictionary
### Feature request

Tokenizers that need a vocab.json file are expected to be created in this manner:
`transformers.Wav2Vec2CTCTokenizer(vocab_file="path/to/vocab.json")`
With the file read with a `json.load` statement.

I'm suggesting an optional  `vocab` parameter  that could be passed instead.

```
vocab= {"a":0, "b":1,......}
tokenizer = transformers.Wav2Vec2CTCTokenizer(vocab=vocab)

# Would also be possible
transformers.Wav2Vec2CTCTokenizer(vocab_file="path/to/vocab.json")
````

### Motivation

Remove the necesity to clutter the disk with a vocab file and allow a dynamic vocab creation process

### Your contribution

I could implement this if it seems like a good addition
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.

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