

True or 'longest_first': truncate to a maximum length specified by the max_length argument or
    the maximum length accepted by the model if no max_length is provided (max_length=None). This will
    truncate token by token, removing a token from the longest sequence in the pair until the proper length is
    reached.
'only_second': truncate to a maximum length specified by the max_length argument or the maximum
    length accepted by the model if no max_length is provided (max_length=None). This will only truncate
    the second sentence of a pair if a pair of sequences (or a batch of pairs of sequences) is provided.
'only_first': truncate to a maximum length specified by the max_length argument or the maximum
    length accepted by the model if no max_length is provided (max_length=None). This will only truncate
    the first sentence of a pair if a pair of sequences (or a batch of pairs of sequences) is provided.
False or 'do_not_truncate': no truncation is applied. This is the default behavior.