fix blip2config int8 error to serialize json


# What does this PR do?

Running the following using transformers=4.30.0.dev0
```
bnb_config = BitsAndBytesConfig(
    load_in_4bit=True,
    bnb_4bit_use_double_quant=True,
    bnb_4bit_quant_type="nf4",
    bnb_4bit_compute_dtype=torch.bfloat16
)
model = AutoModelForVision2Seq.from_pretrained("Salesforce/blip2-opt-2.7b", quantization_config=bnb_config, device_map='auto')
print(model.config)
```
Will give the following error:
```
TypeError: Object of type BitsAndBytesConfig is not JSON serializable
```
Solution add convert BitsAndBytesConfig to json in Blip2Config



## Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

@amyeroberts
@NielsRogge 

<!-- Your PR will be replied to more quickly if you can figure out the right person to tag with @

 If you know how to use git blame, that is the easiest way, otherwise, here is a rough guide of **who to tag**.
 Please tag fewer than 3 people.

Models:

- text models: @ArthurZucker and @younesbelkada
- vision models: @amyeroberts
- speech models: @sanchit-gandhi
- graph models: @clefourrier

Library:

- flax: @sanchit-gandhi
- generate: @gante
- pipelines: @Narsil
- tensorflow: @gante and @Rocketknight1
- tokenizers: @ArthurZucker
- trainer: @sgugger

Integrations:

- deepspeed: HF Trainer/Accelerate: @pacman100
- ray/raytune: @richardliaw, @amogkam

Documentation: @sgugger, @stevhliu and @MKhalusova

HF projects:

- accelerate: [different repo](https://github.com/huggingface/accelerate)
- datasets: [different repo](https://github.com/huggingface/datasets)
- diffusers: [different repo](https://github.com/huggingface/diffusers)
- rust tokenizers: [different repo](https://github.com/huggingface/tokenizers)

Maintained examples (not research project or legacy):

- Flax: @sanchit-gandhi
- PyTorch: @sgugger
- TensorFlow: @Rocketknight1

 -->

cc @younesbelkada <|||||>The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_24094). All of your documentation changes will be reflected on that endpoint.<|||||>I ran `pip install -e .` on transformer main branch 8b169142f8b5735f25ad81313ee382350161d993
Then ran the code snippet and gave the error: 
`TypeError: Object of type BitsAndBytesConfig is not JSON serializable`

Adding my commit will fix this

print screen attached
![install0](https://github.com/huggingface/transformers/assets/9553458/1a95e026-fbfb-4eac-8454-bcdb35b86888)

![error0](https://github.com/huggingface/transformers/assets/9553458/f77f56ce-341e-4fff-a2c4-fb263539c718)


<|||||>Hi @Andrechang 
Again thanks very much for flagging the issue, I realized that you have flagged an issue that is quite important to fix, therefore I made https://github.com/huggingface/transformers/pull/24137 and added you as a co-author, will merge that PR and I will close this one. Feel free to re-open it if you think that the issue is not resolved. Again thanks a lot!<|||||>Thank you for checking and for fix
