[`config.to_dict()`] update test and models that have a composition
# What does this PR do?

Adresses #23876

Most of the configurations that are `composition` are not json serialzable. 
- A test exists, but is usually not run (`ConfigTester` scarcly used on the full config) 
- The test does test the case when `torch_dtype = torch.float16`
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_23888). All of your documentation changes will be reflected on that endpoint.<|||||>3 model config that were not tested need a better update to pass the `test_config`, will work on this once I am back from lunch! 
- blip2
- pix2struct
- clap