class_docstr: Manages the status/heartbeat monitor for a set of workers.
class_docstr_tok: ["Manages","the","status\/heartbeat","monitor","for","a","set","of","workers","."]
code: def num_workers(self):
      return len(self._devices)
  
imports: import threading
  import time
  from google.protobuf import text_format
  from tensorflow.core.protobuf import config_pb2
  from tensorflow.core.util import event_pb2
  from tensorflow.python.client import session
  from tensorflow.python.framework import dtypes
  from tensorflow.python.framework import errors
  from tensorflow.python.framework import ops
  from tensorflow.python.ops import array_ops
  from tensorflow.python.platform import tf_logging
  from tensorflow.python.tpu.ops import tpu_ops
  from tensorflow.python.training import session_run_hook
  from tensorflow.python.training import training_util
label_desc: Use image node_id 3 for calling the WorkerHeartbeatManager obj's underlying member method code with example usage: obj.num_workers() and returns: len