XLMR tokenizer is fully picklable
# What does this PR do?
This addresses the issue here https://github.com/huggingface/transformers/issues/13200 to summarize:
- unpickling was dependant on what was on disk
the tokenizer is now unpickled only with the serialised proto. 

This is needed if you want to write a pyspark udf which tokenizes a column, as the tokenizer needs to be pickled and sent to other nodes.
# Who can help
@LysandreJik 
> This looks good to me - do you think you could implement a test in `tests/test_tokenization_xlm_roberta.py`?

done