    def truncate_sequences(self,
                           ids,
                           pair_ids=None,
                           num_tokens_to_remove=0,
                           truncation_strategy='longest_first',
                           stride=0):
        """
        Truncates a sequence pair in place to the maximum length.

        Args:
            ids: list of tokenized input ids. Can be obtained from a string by chaining the
                `tokenize` and `convert_tokens_to_ids` methods.
            pair_ids: Optional second list of input ids. Can be obtained from a string by chaining the
                `tokenize` and `convert_tokens_to_ids` methods.
            num_tokens_to_remove (:obj:`int`, `optional`, defaults to ``0``):
                number of tokens to remove using the truncation strategy
            truncation_strategy: string selected in the following options:
                - 'longest_first' (default) Iteratively reduce the inputs sequence until the input is under max_seq_len
                    starting from the longest one at each token (when there is a pair of input sequences).
                    Overflowing tokens only contains overflow from the first sequence.
                - 'only_first': Only truncate the first sequence. raise an error if the first sequence is shorter or equal to than num_tokens_to_remove.
                - 'only_second': Only truncate the second sequence
                - 'do_not_truncate': Does not truncate (raise an error if the input sequence is longer than max_seq_len)
            stride (:obj:`int`, `optional`, defaults to ``0``):
                If set to a number along with max_seq_len, the overflowing tokens returned will contain some tokens
                from the main sequence returned. The value of this argument defines the number of additional tokens.
        """
        if num_tokens_to_remove <= 0:
            return ids, pair_ids, []

        if truncation_strategy == 'longest_first':
            overflowing_tokens = []
            for _ in range(num_tokens_to_remove):
                if pair_ids is None or len(ids) > len(pair_ids):
                    overflowing_tokens = [ids[-1]] + overflowing_tokens
                    ids = ids[:-1]
                else:
                    pair_ids = pair_ids[:-1]
            window_len = min(len(ids), stride)
            if window_len > 0:
                overflowing_tokens = ids[-window_len:] + overflowing_tokens
        elif truncation_strategy == 'only_first':
            assert len(ids) > num_tokens_to_remove
            window_len = min(len(ids), stride + num_tokens_to_remove)
            overflowing_tokens = ids[-window_len:]
            ids = ids[:-num_tokens_to_remove]
        elif truncation_strategy == 'only_second':
            assert pair_ids is not None and len(pair_ids) > num_tokens_to_remove
            window_len = min(len(pair_ids), stride + num_tokens_to_remove)
            overflowing_tokens = pair_ids[-window_len:]
            pair_ids = pair_ids[:-num_tokens_to_remove]
        elif truncation_strategy == 'do_not_truncate':
            raise ValueError(
                "Input sequence are too long for max_length. Please select a truncation strategy."
            )
        else:
            raise ValueError(
                "Truncation_strategy should be selected in ['longest_first', 'only_first', 'only_second', 'do_not_truncate']"
            )
        return (ids, pair_ids, overflowing_tokens)
Truncates a sequence pair in place to the maximum length.

Args:
ids: list of tokenized input ids. Can be obtained from a string by chaining the
`tokenize` and `convert_tokens_to_ids` methods.
pair_ids: Optional second list of input ids. Can be obtained from a string by chaining the
`tokenize` and `convert_tokens_to_ids` methods.
num_tokens_to_remove (:obj:`int`, `optional`, defaults to ``0``):
number of tokens to remove using the truncation strategy
truncation_strategy: string selected in the following options:
- 'longest_first' (default) Iteratively reduce the inputs sequence until the input is under max_seq_len
starting from the longest one at each token (when there is a pair of input sequences).
Overflowing tokens only contains overflow from the first sequence.
- 'only_first': Only truncate the first sequence. raise an error if the first sequence is shorter or equal to than num_tokens_to_remove.
- 'only_second': Only truncate the second sequence
- 'do_not_truncate': Does not truncate (raise an error if the input sequence is longer than max_seq_len)
stride (:obj:`int`, `optional`, defaults to ``0``):
If set to a number along with max_seq_len, the overflowing tokens returned will contain some tokens
from the main sequence returned. The value of this argument defines the number of additional tokens.
ptstructure/vqa/pytorchnlp/transformers/tokenizer_utils.py