code: def add_observer(self, observer: Observer):
      observing_keys = observer.get_observing_keys()
      unknown_keys = [
          k
          for k in observing_keys
          if k not in self._observable_value_types
      ]
      if unknown_keys:
          logger.warning(
              f"{unknown_keys} cannot be observed in {type(self)}"
          )
      for k in observing_keys:
          if (
              k in self._observers
              and observer not in self._observers[k]
          ):
              self._observers[k].append(observer)
      return self
  
file_path: ReAgent/reagent/core/tracker.py
imports: import functools
  import logging
  from typing import Dict, List, Type
  import torch
label_desc: Use image node_id 3 for calling the ObservableMixin obj's underlying member method code with example usage: obj.add_observer(observer) and returns: self