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: Log performance metrics every log_freq batches, if log_freq is not None.
docstr_tok: ["Log","performance","metrics","every","log_freq","batches",",","if","log_freq","is","not","None","."]
code: def on_loss_and_meter(self, task: "tasks.ClassyTask") -> None:
      """
      Log performance metrics every log_freq batches, if log_freq is not None.
      """
      if self.log_freq is None:
          return
      batches = len(task.losses)
      if batches and batches % self.log_freq == 0:
          self._log_performance_metrics(task)
  
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 3 for calling the LogPerfTimeMetricsHook obj's underlying member method code with example usage: obj.on_loss_and_meter(task) without return types