Managing special tokens (like mask, beginning-of-sentence, etc.): adding them, assigning them to attributes in the
  tokenizer for easy access and making sure they are not split during tokenization.
[BatchEncoding] holds the output of the
[~tokenization_utils_base.PreTrainedTokenizerBase]'s encoding methods (__call__,
encode_plus and batch_encode_plus) and is derived from a Python dictionary. When the tokenizer is a pure python
tokenizer, this class behaves just like a standard python dictionary and holds the various model inputs computed by
these methods (input_ids, attention_mask...). When the tokenizer is a "Fast" tokenizer (i.e., backed by
HuggingFace tokenizers library), this class provides in addition
several advanced alignment methods which can be used to map between the original string (character and words) and the
