Converting all model Config classes to dataclasses

It seems that we could save a lot of boilerplate code and potentially prevent some bugs if we migrated all of the model config classes over to being dataclasses. Already many of our classes (BaseModelOutput, TrainingArguments, etc.) are dataclasses, so we are already committed to having dataclasses as a dependency.

It's relatively low priority, but I would be willing to help implement the change since I'm kind of a neat freak about code.