tokenizer.Tokenizer compatibility with Inference API or Auto* classes

# 🚀 Feature request
Make tokenizers created with the tokenizers library compatible with the Inference API or Auto classes

## Motivation

I have trained a model on a specific domain by modeling a sequence generation problem as a language modeling problem to predict the next token in the set. The tokenizer associated with the model I used (TransformerXL) was not compatible with my domain since my tokens contained whitespace so I created my own using the `WordLevelTrainer` class in the `tokenizers` library. **Now that I have a complete working solution I would like to use this tokenizer and model in the huggingface Inference API, however it does not work because it requires the tokenizer associated with the model**. Making the `transformers` models compatible with `tokenizers` library could make all kinds of use cases outside of NLP possible with these libraries.

## Your contribution

Is it possible to hack the saved config for a tokenizer created through the `tokenizers` library to work directly with the `Auto` classes? If so I can document this approach for other users.