class_docstr: Computes and prints performance metrics. Logs at the end of a phase
  or every log_freq if specified by user.
class_docstr_tok: ["Computes","and","prints","performance","metrics",".","Logs","at","the","end","of","a","phase","or","every","log_freq","if","specified","by","user","."]
docstr: Args:
      log_freq: if specified, logs every log_freq batches also.
docstr_tok: ["Args",":","log_freq",":","if","specified",",","logs","every","log_freq","batches","also","."]
code: def __init__(self, log_freq: Optional[int] = None) -> None:
      """
      Args:
          log_freq: if specified, logs every log_freq batches also.
      """
      super().__init__()
      self.log_freq: Optional[int] = log_freq
      self.start_time: Optional[float] = None
  
imports: import atexit
  import datetime
  import json
  import logging
  import time
  from typing import List, Optional
  import torch
  from classy_vision import meters, tasks
  from classy_vision.generic.distributed_util import get_rank, is_primary
  from classy_vision.hooks.classy_hook import ClassyHook
  from fairscale.nn.data_parallel import FullyShardedDataParallel
  from iopath.common.file_io import g_pathmgr
  from vissl.models.model_helpers import model_output_has_nan
  from vissl.utils.checkpoint import CheckpointWriter, is_checkpoint_phase
  from vissl.utils.env import get_machine_local_and_dist_rank
  from vissl.utils.io import save_file
  from vissl.utils.logger import log_gpu_stats
  from vissl.utils.perf_stats import PerfStats
label_desc: Use image node_id 1 to create a new LogPerfTimeMetricsHook object from inherited base classes: ClassyHook with example: obj = LogPerfTimeMetricsHook(log_freq)