Correct model_type in PretrainedConfig's to_dict

[fxmarty]

As per title, now

from transformers import AutoConfig, PretrainedConfig

cfg = AutoConfig.from_pretrained("bert-base-uncased")
config = PretrainedConfig.from_dict(cfg.to_dict())
config.model_type = "my-model"

print(config.to_dict()["model_type"])

rightfully yields my-model, while it use to give "" (the class attribute value of PretrainedConfig).

I think instance attributes (if any) should take precedence over class attributes.

Related to huggingface/optimum#1645

[amyeroberts]

Thanks for opening this PR @fxmarty!

The reason this happens at the moment is that model_type is intended to be a class attribute, not an instance attribute i.e. to be consistent and shared for all instances of the config object and not modified instance to instance. What's the motivation for modifying it like this?

[fxmarty]

Understood!