class_docstr: Track free RAM on the host.
  
  When triggered, it writes the size of free RAM into monitors.
class_docstr_tok: ["Track","free","RAM","on","the","host",".","When","triggered",",","it","writes","the","size","of","free","RAM","into","monitors","."]
code: def _setup_graph(self):
      logger.info(
          "[HostMemoryTracker] Free RAM in setup_graph() is {:.2f} GB.".format(
              self._free_ram_gb()
          )
      )
  
imports: import multiprocessing
  import numpy
  import os
  import time
  import tensorflow
  from six.moves import map, queue
  import psutil
  from ..tfutils.common import gpu_available_in_session
  from ..utils import logger
  from ..utils.timer import Timer
  from ..utils.concurrency import ensure_proc_terminate, start_proc_mask_signal
  from ..utils.gpu import get_num_gpu
  from ..utils.nvml import NVMLContext
  from .base import Callback
label_desc: Use image node_id 1 for calling the HostMemoryTracker obj's underlying member method code with example usage: obj._setup_graph() without return types