Tokenizer not loaded for image-to-text pipelines when specified
### System Info

Transformers 4.23.1

### Who can help?

@Narsil 

### Information

- [ ] The official example scripts
- [ ] My own modified scripts

### Tasks

- [ ] An officially supported task in the `examples` folder (such as GLUE/SQuAD, ...)
- [ ] My own task or dataset (give details below)

### Reproduction

```python
from transformers import pipeline
nlp = pipeline("image-to-text", model="ydshieh/vit-gpt2-coco-en", tokenizer="ydshieh/vit-gpt2-coco-en")
nlp("image.jpg")
```

Results in:

```
  File "transformers/pipelines/image_to_text.py", line 89, in postprocess
    "generated_text": self.tokenizer.decode(
AttributeError: 'str' object has no attribute 'decode'
```

### Expected behavior

Caption to be returned.

It appears something around this line is being tripped up - https://github.com/huggingface/transformers/blob/main/src/transformers/pipelines/__init__.py#L733
cc @Narsil <|||||>If that is an unexpected issue, I can debug it and try to fix. <|||||>Hi @davidmezzetti ,

The "string" is not resolved in `pipeline` when both `model` and `tokenizer` are sent.
It should work when you send only 1.

You could definitely create a PR to try and resolve `tokenizer` too in that situation.
It's tricky magic code so be careful.<|||||>Thanks @narsil. In reviewing the way I'm calling pipelines, there is no reason to pass both a `model` and `tokenizer` as they are the same.

It appears that this line: https://github.com/huggingface/transformers/blob/main/src/transformers/pipelines/__init__.py#L737 could be updated to also check if the tokenizer is a (str, tuple) for multi models. But it seems like a highly niche/possibly unnecessary use case. <|||||>`pipeline` is MAGICAL by nature, I'm not against adding even more magic to it. As long as the actual pipeline classes, stay much more down to earth and less magical. 

Magic is super nice when it works, but much harder to work with/evolve when it doesn't :)<|||||>Sounds good, I'll go ahead and close this issue. 