code: def __len__(self):
      self._check_annotation_db_present()
      return len(self.annotation_db)
  
file_path: mmf/mmf/datasets/databases/features_database.py
imports: import logging
  from multiprocessing.pool import ThreadPool
  import tqdm
  from mmf.datasets.databases.image_database import ImageDatabase
  from mmf.datasets.databases.readers.feature_readers import FeatureReader
  from mmf.utils.distributed import is_main
  from mmf.utils.general import get_absolute_path
label_desc: Use image node_id 6 for calling the FeaturesDatabase obj's underlying member method code with example usage: obj.__len__() and returns: len