class_docstr: UnmanagedTrialHeartbeat updates the state on context enter/exit.
code: def start(self) -> "_Heartbeat":
      self._update_state(bindings.trialv1State.RUNNING)
      self._heartbeat = _HeartbeatReporter(
          self._session, self._trial_id
      )
      self._heartbeat.start()
  
      self._hook = _ExitHook()
      self._hook.hook()
      atexit.register(self._exit_handler)
  
      return self
  
imports: import atexit
  import logging
  import sys
  import threading
  import time
  import types
  from typing import Any, Optional, Union
  from determined.common import api
  from determined.common.api import bindings
label_desc: Use image node_id 1 for calling the _UnmanagedTrialHeartbeat obj's underlying member method code with example usage: obj.start() and returns: self