Delete key or set to `None` in __getstate__ impl.
Hi,
there are some places that implement `__getstate__` because an object has a reference to an other object that is not pickable.

`__getstate__` then "deletes" the reference by setting it to `None`. Just a few examples:

https://github.com/huggingface/transformers/blob/73fde1defe9be259a47b9024525882f3ec420994/src/transformers/models/m2m_100/tokenization_m2m_100.py#L272

https://github.com/huggingface/transformers/blob/73fde1defe9be259a47b9024525882f3ec420994/src/transformers/models/marian/tokenization_marian.py#L299

IMO it would be better to delete the keys instead of setting them to `None`. Like this: `del state["sp_model"]`

What do you think @sgugger @LysandreJik ? I can provide a PR if wanted.
Hmmm - thinking about it - the value is also set to None by default in the constructor. I will close this...