class_docstr: Add Prefix to src (& tgt) sentence.
docstr: Warm up to get prefix dictionary.
docstr_tok: ["Warm","up","to","get","prefix","dictionary","."]
code: def warm_up(self, vocabs=None):
      """Warm up to get prefix dictionary."""
      super().warm_up(None)
      self.prefix_dict = self.get_prefix_dict(self.opts)
  
imports: from onmt.utils.logging import logger
  from onmt.transforms import register_transform
  from .transform import Transform, ObservableStats
label_desc: Use image node_id 6 for calling the PrefixTransform obj's underlying member method code with example usage: obj.warm_up(vocabs) without return types