Fix resize_token_embeddings for Transformer-XL (#4759)
* Fixed resize_token_embeddings for transfo_xl model
* Fixed resize_token_embeddings for transfo_xl.
Added custom methods to TransfoXLPreTrainedModel for resizing layers of
the AdaptiveEmbedding.
* Updated docstring
* Fixed resizinhg cutoffs; added check for new size of embedding layer.
* Added test for resize_token_embeddings
* Fixed code quality
* Fixed unchanged cutoffs in model.config
Co-authored-by: Rafael Weingartner <rweingartner.its-b2015@fh-salzburg.ac.at>
