Allow per-version configurations
Similarly to https://github.com/huggingface/transformers/pull/12713, this allows per-version configurations. This is necessary for LayoutXLM, which up to now was using the configuration-defined `XLMRobertaTokenizer`, but which should now use the `LayoutXLMTokenizer`.

Updating the configuration would mean breaking all previous versions of `transformers` that were using LayoutXLM. Not updating this parameter means that LayoutXLM will never benefit from `LayoutXLMTokenizer` through the `AutoTokenizer` API.

Resolves https://github.com/huggingface/transformers/issues/14275

This implements similar tests to the tokenizer, but instead of using `bert-base-cased`, it uses the actual model that is at issue (`microsoft/layoutxlm-base`). This model should continue using the `XLMRobertaTokenizer` until a new minor version is released, as the configuration I uploaded is named `config.4.13.0.json`: https://huggingface.co/microsoft/layoutxlm-base/blob/main/config.4.13.0.json