Make sure custom configs work with Transformers
# What does this PR do?

This PR makes sure that once can use a custom configuration in Transformers with no trouble in the `save_pretrained`/`from_pretrained` methods. The only requirement is that it must accept kwargs as the `from_pretrained` method calls the init with lots of them (this will be documented soon in a PR that shows how to write a custom model and config).

An alternative would be to make sure that every subclass of `PretrainedConfig` calls the `super().__init__` class with a check like PyTorch, but it would force us to put that calls at the beginning of each init before any attribute is set. This PR opts for a more lenient strategy, and makes sure any config attribute we try to reach is done through a `getattr`.
_The documentation is not available anymore as the PR was closed or merged._