Accumulate tokens into batches in `PreTrainedTokenizerBase.add_tokens()` (#17119)
* Accumulate tokens into batches in PreTrainedTokenizerBase.add_tokens()
For tokenizers with a small number of special tokens or special tokens
with consecutive token IDs, this reduces the time complexity of creating
the trie from quadratic to linear, see also #16936.
* Extend explanation of batching added tokens