class_docstr: Caches actor classes.
  
  ray.remote is a registration call. It sends the serialized object to the
  key value store (redis), and will be fetched at an arbitrary worker
  later. Registration does not use any Ray scheduling resources.
  
  Later, class.remote() actually creates the remote actor. The
  actor will be instantiated on some arbitrary machine,
  according to the underlying Ray scheduler.
  
  Without this cache, you would register the same serialized object
  over and over again. Naturally, since redis doesn’t spill to disk,
  this can easily nuke the redis instance (and basically blow up Ray).
  This cache instead allows us to register once and only once.
  
  Note that we assume there can be multiple trainables in the
  system at once.
code: def __init__(self):
      self._cache = {}
  
imports: import os
  import ray
  from ray.air.constants import COPY_DIRECTORY_CHECKPOINTS_INSTEAD_OF_MOVING_ENV
  from ray.train.constants import RAY_CHDIR_TO_TRIAL_DIR
label_desc: Use image node_id 1 to create a new _ActorClassCache object  with example: obj = _ActorClassCache()