AutoTokenizer.from_pretrained function cannot be customized 
A customized tokenizer has been provided in the tokenizer library, so that I can directly use the word segmentation data and vocab in the previous fairseq, but there is AutoTokenizer.from_pretrained function in the transformer, which cannot be customized like the tokenizer library, so I have no choice to directly use fairseq's vocab and word segmentation data in transformer. 

What needs to be done？
Hi! Could you provide an example of the usage that you would like to see with the `transformers` library, so that we may see what can be done?<|||||>Is this the same as #8125 ?<|||||>This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
