docstr: Gets called at the beginning of each epoch.
  Arguments
  ---------
  stage : sb.Stage
      One of sb.Stage.TRAIN, sb.Stage.VALID, or sb.Stage.TEST.
  epoch : int
      The currently-starting epoch. This is passed
      `None` during the test stage.
code: def on_stage_start(self, stage, epoch=None):
      """Gets called at the beginning of each epoch.
      Arguments
      ---------
      stage : sb.Stage
          One of sb.Stage.TRAIN, sb.Stage.VALID, or sb.Stage.TEST.
      epoch : int
          The currently-starting epoch. This is passed
          `None` during the test stage.
      """
  
      # Set up statistics trackers for this stage
      self.loss_metric = sb.utils.metric_stats.MetricStats(
          metric=sb.nnet.losses.nll_loss
      )
  
      # Set up evaluation-only statistics trackers
      if stage != sb.Stage.TRAIN:
          self.error_metrics = self.hparams.error_stats()
  
file_path: speechbrain/recipes/IEMOCAP/emotion_recognition/train_with_wav2vec2.py
imports: import os
  import sys
  import speechbrain
  from hyperpyyaml import load_hyperpyyaml
label_desc: Use image node_id 3 for calling the EmoIdBrain obj's underlying member method code with example usage: obj.on_stage_start(stage, epoch) without return types