    def extend_embedder_vocab(self, embedding_sources_mapping: Dict[str, str] = None) -> None:
        """
        Iterates through all embedding modules in the model and assures it can embed
        with the extended vocab. This is required in fine-tuning or transfer learning
        scenarios where model was trained with original vocabulary but during
        fine-tuning/transfer-learning, it will have it work with extended vocabulary
        (original + new-data vocabulary).

        # Parameters

        embedding_sources_mapping : `Dict[str, str]`, optional (default = `None`)
            Mapping from model_path to pretrained-file path of the embedding
            modules. If pretrained-file used at time of embedding initialization
            isn't available now, user should pass this mapping. Model path is
            path traversing the model attributes upto this embedding module.
            Eg. "_text_field_embedder.token_embedder_tokens".
        """
        # self.named_modules() gives all sub-modules (including nested children)
        # The path nesting is already separated by ".": eg. parent_module_name.child_module_name
        embedding_sources_mapping = embedding_sources_mapping or {}
        for model_path, module in self.named_modules():
            if hasattr(module, "extend_vocab"):
                pretrained_file = embedding_sources_mapping.get(model_path)
                module.extend_vocab(
                    self.vocab,
                    extension_pretrained_file=pretrained_file,
                    model_path=model_path,
                )
Iterates through all embedding modules in the model and assures it can embed
with the extended vocab. This is required in fine-tuning or transfer learning
scenarios where model was trained with original vocabulary but during
fine-tuning/transfer-learning, it will have it work with extended vocabulary
(original + new-data vocabulary).

# Parameters

embedding_sources_mapping : `Dict[str, str]`, optional (default = `None`)
Mapping from model_path to pretrained-file path of the embedding
modules. If pretrained-file used at time of embedding initialization
isn't available now, user should pass this mapping. Model path is
path traversing the model attributes upto this embedding module.
Eg. "_text_field_embedder.token_embedder_tokens".
allennlp/models/model.py