class_docstr: subword_nmt: official BPE subword transform class.
docstr: Do bpe subword tokenize.
docstr_tok: ["Do","bpe","subword","tokenize","."]
code: def tokenize_string(self, string, side="src", is_train=False):
      """Do bpe subword tokenize."""
      tokens = string.split(" ")
      bpe_model = self.load_models[side]
      dropout = self.dropout[side] if is_train else 0.0
      segmented = bpe_model.segment_tokens(tokens, dropout=dropout)
      return segmented
  
imports: import re
  from onmt.utils.logging import logger
  from onmt.transforms import register_transform
  from .transform import Transform, ObservableStats
  from onmt.constants import DefaultTokens
label_desc: Use image node_id 5 for calling the BPETransform obj's underlying member method code with example usage: obj.tokenize_string(string, side, is_train) and returns: segmented