docstr: Args:
      name (str): the name of the dataset split, e.g. "coco_train2017"
      key: the key of the metadata, e.g., "class_names"
  
  Returns:
      value
code: def get_metadata(name, key):
      """
      Args:
          name (str): the name of the dataset split, e.g. "coco_train2017"
          key: the key of the metadata, e.g., "class_names"
  
      Returns:
          value
      """
      return DatasetRegistry._metadata_registry[name][key]
  
file_path: tensorpack/examples/FasterRCNN/dataset/dataset.py
imports: from collections import defaultdict
label_desc: Use image node_id 4 for calling the DatasetRegistry obj's underlying member method code with example usage: obj.get_metadata(name, key) and returns: DatasetRegistry