message: Enable MWMS check health
  This enables check health feature in MultiWorkerMirroredStrategy. With this we should be able to more reliably detect peer failures and avoid hanging. The support for NCCL is not done yet, but it's still better than not having it.
  Note that this only works for eager.
diff: @@ -184,7 +184,7 @@ class CollectiveAllReduceExtended(mirrored_strategy.MirroredExtended):
  # Whether to perdically check the health of the cluster. If any worker is not
  # reachable, collectives are aborted and the user program should get a
  # tf.errors.UnavailableError. It's required to restart in order to recover.
  - _enable_check_health = False
  + _enable_check_health = True
  # Check health interval in seconds.
  _check_health_interval = 30
  # Timeout in seconds for the first check health. The first check health needs
  @@ -387,7 +387,6 @@ class CollectiveAllReduceExtended(mirrored_strategy.MirroredExtended):
  self._rpc_layer = cluster_resolver.rpc_layer
  self._warn_nccl_no_gpu()
  - # TODO(b/151232436): Enable check health thread by default.
  if self._enable_check_health:
  self._start_check_health_thread()
  