code: def _init_db(self):
      self.env = lmdb.open(
          self.db_path,
          subdir=os.path.isdir(self.db_path),
          readonly=True,
          lock=False,
          readahead=False,
          meminit=False,
      )
      with self.env.begin(write=False, buffers=True) as txn:
          self.image_ids = pickle.loads(txn.get(b"keys"))
          self.image_id_indices = {
              self.image_ids[i]: i
              for i in range(0, len(self.image_ids))
          }
  
imports: import math
  import os
  import pickle
  from typing import Any
  import lmdb
  import numpy
  import torch
  from mmf.utils.file_io import PathManager
label_desc: Use image node_id 2 for calling the LMDBFeatureReader obj's underlying member method code with example usage: obj._init_db() without return types