Get word seperator char for tokenization
# 🚀 Feature request
When using some of tokenization models (e.g. : CamemBERT) you can find the char used to separate words after some investigation ([start](https://github.com/huggingface/transformers/blob/a4c75f149269099a98613f51b76cd0b579a109ee/src/transformers/tokenization_camembert.py#L274), [first jump](https://github.com/huggingface/transformers/blob/master/src/transformers/tokenization_camembert.py#L27), [last jump](https://github.com/huggingface/transformers/blob/master/src/transformers/tokenization_xlnet.py#L43)). Making it a general rule seems interesting.

## Motivation

With this you can detect whether a token is a subword or the start of a word (and then use this info for masking). But this is inconsistent across models. After several hours I couldn't find this information for RoBERTa (and GPT2) in the code. We can obviously get this char experimentally but it is not robust to new models or even different versions of the model. 

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.
<|||||>I don't think you should delete the issue since it's kinda useful to have and not having it may be embarassing. But do as you want !<|||||>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.
