Declare __len__ method in PreTrainedTokenizerBase
# What does this PR do?

When type hinting a tokenizer with `PreTrainedTokenizerBase`, it doesn't know supprt `len(tokenizer)`, but both slow and fast version implement `__len__` so we declare it at least to make the type hints happy, we could make this an `abstract_method` as well if needed
_The documentation is not available anymore as the PR was closed or merged._